26-Variable Volume and Temperature Systems (TDP-704).pdf

COMMERCIAL HVAC SYSTEMS

Variable Volume and Temperature

Technical Development Program

Technical Development Programs (TDP) are modules of technical training on HVAC theory, system design, equipment selection and application topics. They are targeted at engineers and designers who wish to develop their knowledge in this field to effectively design, specify, sell or apply HVAC equipment in commercial applications. Although TDP topics have been developed as stand-alone modules, there are logical groupings of topics. The modules within each group begin at an introductory level and progress to advanced levels. The breadth of this offering allows for customization into a complete HVAC curriculum – from a complete HVAC design course at an introductory-level or to an advanced-level design course. Advanced-level modules assume prerequisite knowledge and do not review basic concepts.

VVT is an economical, all-air zoned system that is ideal for many commercial jobs, especially at a time when there is so much design emphasis being placed on high-quality air treatment, outdoor air ventilation, and room air circulation. VVT systems are a popular solution for heating and cooling multiple zone applications in small to medium size buildings. VVT controls typically are supplied pre-packaged from the HVAC equipment supplier and are ready to install by the mechanical contractor. Many manufacturers offer VVT-type systems. These systems are highly dependent on the control hardware and software used. This TDP uses the Carrier VVT system for all examples. The objective of this module is to define VVT, identify applications, compare it to alternative systems, and describe how it achieves zone temperature control.

© 2004 Carrier Corporation. All rights reserved. The information in this manual is offered as a general guide for the use of industry and consulting engineers in designing systems. Judgment is required for application of this information to specific installations and design applications. Carrier is not responsible for any uses made of this information and assumes no responsibility for the performance or desirability of any resulting system design. The information in this publication is subject to change without notice. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, for any purpose, without the express written permission of Carrier Corporation.

Printed in Syracuse, NY CARRIER CORPORATION Carrier Parkway Syracuse, NY 13221, U.S.A.

Table of Contents Introduction...................................................................................................................................... 1 The VVT System ............................................................................................................................. 4 VVT is Variable Volume ............................................................................................................. 6 VVT is Variable Temperature ..................................................................................................... 6 What is Zoning?............................................................................................................................... 7 Types of VVT Jobs .......................................................................................................................... 8 Jobs at 25 Tons or Less................................................................................................................ 8 Jobs Larger than 25 Tons............................................................................................................. 9 Retrofitting Existing Systems with VVT ................................................................................... 10 VVT versus Other Systems............................................................................................................ 13 VVT Advantages ....................................................................................................................... 14 VAV System Comparisons ........................................................................................................ 16 VVT versus Multiple Units........................................................................................................ 18 Zoning the Building for VVT ........................................................................................................ 19 Basic Sequence of Operation ......................................................................................................... 22 Linkage ...................................................................................................................................... 23 Pressure Dependent (PD) versus Pressure Independent (PI) ..................................................... 23 Call for Heat/Cool and Equipment Mode .................................................................................. 24 System Changeover ................................................................................................................... 25 Selecting Zone Priority - Reference Zone.................................................................................. 26 Fan Sequence of Operation........................................................................................................ 26 VVT Air Distribution System Design............................................................................................ 27 Sealing VVT Ducts .................................................................................................................... 30 Dampers ..................................................................................................................................... 31 Round Dampers ..................................................................................................................... 32 Rectangular Dampers............................................................................................................. 32 Bypass System Layout............................................................................................................... 32 Bypass Components............................................................................................................... 33 Functionality .......................................................................................................................... 33 Layout .................................................................................................................................... 34 Damper Sizing ....................................................................................................................... 36 Diffuser Layout.......................................................................................................................... 37 Control System Details .................................................................................................................. 40 Linkage Coordinator versus Standard Zone Controllers............................................................ 40 Bypass Controller ...................................................................................................................... 41 The System Pilot........................................................................................................................ 41 Space Sensor Locations and Options......................................................................................... 42 Combined Space Temperature and CO2 Sensing....................................................................... 43 Humidity Sensor ........................................................................................................................ 43 Zone Sensor Averaging.............................................................................................................. 43 Outside Air Temperature Sensor ............................................................................................... 43 Zone Level Demand Controlled Ventilation (DCV).................................................................. 44 Zoning Systems with DCV .................................................................................................... 44 Wiring and Power Requirements ........................................................................................... 45 System Options ...................................................................................................................... 45 Supplemental and Perimeter Heat.................................................................................................. 46

Summary ........................................................................................................................................49 Work Session .................................................................................................................................50 Designer Checklist .....................................................................................................................52 Engineering Design Steps ......................................................................................................52 Installation Notes for Contractors ..............................................................................................54 VVT Installation Start-up Request Checklist.............................................................................56 Work Session Answers ..............................................................................................................58

VARIABLE VOLUME AND TEMPERATURE

Introduction VVT (variable volume and temperature) is an economical, all-air zoned system that is ideal for many commercial jobs, especially at a time when there is so much design emphasis being placed on high quality air treatment, outdoor air ventilation, and room air circulation. When a single heating/cooling unit is used, VVT works well for systems up to about 25 tons of total cooling capacity. Multiple systems make its application practical for much larger jobs. This module defines VVT and describes how it achieves zone temperature control. Applications for the system will be identified and VVT will be compared with alternative systems. Since the operation of the VVT system is under the direction of a complete, factory-packaged DDC (direct digital control) control system, various pre-programmed, operational sequences will be described so that the way it works will be clear. Guidelines for VVT system design are given so that the designer may focus on some of the unique aspects of the system. Air conditioning design is all about solving building comfort needs to satisfy the occupants of that building. One of the buildings we will use to illustrate zoning and the use of VVT is this manufacturing office, which is a 60’ x 100’ single-story commercial construction attached to a small, air-conditioned electronics manufacturing and assembly factory. This is an owner-occupied office with relatively permanent partition arrangement and an expectation for a reasonably good level of comfort. Occupants will be exposed to the indoor environment for long periods of time, so their comfort expectation will tend to be high. In addition, they are sedentary, for the most part, which increases their sensitivity to variations in Figure 1 temperature, air distribu- Manufacturing Office Example Building tion and air stratification.

Commercial HVAC Systems

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Figure 2 60’ x 100’ Manufacturing Office VVT System Layout


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The task of the air-conditioning system is to maintain comfort in the building by simultaneously controlling space temperature, humidity, air motion, air purity, air quality, and mean radiant temperature. In our case, the system will be a VVT system. The layout shown in Figure 2 includes many details for a real design. In a sense, this is your “map” for the information that is coming. It will help you to focus on the area of the system being addressed in virtually every portion of this module. Take a few minutes to look the layout over, reading the designer’s system comments, which describe the VVT system designed for this job. For this VVT system, a single heating/cooling, constant volume packaged rooftop unit provides central heating or cooling capacity to the VVT boxes. Each box modulates its volume control damper in response to the zone thermostat or sensor. Air not used by the zones is bypassed into the return air ceiling plenum. Thus, the zone airflow is variable but the rooftop airflow is relatively constant. This permits the use of standard constant volume equipment. Each box has a user-defined minimum cfm setting to ensure adequate room air circulation and outdoor air ventilation in the zone regardless of zone load reduction. Typical minimum airflow settings vary from about 10 to 30 percent of design flow and are subject to local codes. The VVT system is designed to provide all cooling capacity centrally and as much central heating as possible. When all zones require some degree of cooling, the unit remains in the cooling mode. When all zones require some degree of heating, the unit remains in the heating mode. However, when both heating and cooling loads occur at the same time, it becomes a time-share system. That is, its electronic controls determine the greatest need (heating or cooling) and they first satisfy that mode centrally. Then, once satisfied, it switches over to the opposite mode. The system can continue switching over from central cooling to central heating, back and forth, to satisfy all zones; thus, the concept of capacity time sharing. Because zone 7 (interior zone) requires year-round cooling whenever occupied and lighted, the unit will need to remain in the cooling mode during most of its occupied cycle. Therefore, all perimeter zone damper units are equipped with a hot water supplementary heater. Electric heaters may be used instead. The supplementary heaters will pick up any zone heating load during the occupied cycle of operation if the central unit is in the cooling mode. The supplementary heaters will be off if the central unit is in the heating mode. The supplementary heaters are deactivated during the unoccupied cycle in both the heating and cooling modes. If a separate system is installed in the zone with an unusual load pattern (zone 7), the energy efficiency of the system will be enhanced at the expense of a more costly installation. Linear slot diffusers are used to keep cold primary air up on the ceiling at the reduced airflow occurring at partial cooling load. Conventional concentric, perforated, or curved-blade diffusers will create dumping of cold supply air on the occupants, causing poor room air mixing and temperature sensing, with the associated customer complaints. Director linear diffusers are used around the perimeter to enhance overhead heating. They contain a heat-sensitive element to change the direction of air diffusion to one-way when warm air is being delivered. That way, warm supply air washes the outside wall, as it should. Conventional, low-velocity, low-pressure sheet metal ductwork is used. It has a 1-in. duct wrap. Pre-insulated flex duct is used for limited lengths to make diffuser connections. All diffuser runouts include a round butterfly balancing damper. Observe local code limitations on flex duct use. The VVT boxes are sized to match the ductwork for ease of installation and fewest fittings.


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The building occupants have comfort needs that the system is designed to solve. The system components provide heat transfer, filtration, ventilation, and air circulation capacity necessary to control the comfort conditions, like air temperature, humidity, cleanliness and distribution in the building spaces. In this module, we will refer to the central equipment as the packaged air handler; air source; HVAC equipment; packaged unit, or rooftop unit. Even though VVT systems typically use packaged rooftop units for their central air source and heating/cooling capacity, VVT can also be applied to a split system with a packaged air handler. The VPAC (vertical packaged air conditioner) is another Figure 3 good air source for VVT, VVT can be used with all types of heating-cooling equipment. since it tends to be applied floor-by-floor for renovating existing buildings, where some zoning would be welcome. In essence, the VPAC is the indoor version of the rooftop unit, since it is a self-contained packaged air handler with all refrigeration cycle components included in one factory-assembled package. The only thing needed for the VPAC is a cooling tower to reject heat from the water leaving the water-cooled condenser at each unit. Air-cooled versions are also available, which reject condenser heat locally, through a wall, window, or by using a remote air-cooled condenser.

The VVT System VVT stands for variable volume and temperature. VVT is provided with a complete factorypackaged control system designed to provide multiple zones of temperature control using a low cost, single zone, constant volume heating and cooling packaged rooftop unit, VPAC, or split system. Packaged rooftop units (RTUs) are most often used. In the past, some manufacturers marketed a dump-box zone terminal that sent supply air that was not needed at the zone to the ceiling plenum return space. Systems using this kind of terminal were called VAV bypass systems. Carrier developed VVT, which uses a bypass concept, but does it at the air handler rather than at the space. It incorporates a complete, factory-designed DDC control system for the entire system instead of merely using dump-box terminals. Today VVT can be applied to air systems using either a ceiling return air plenum or a ducted return.


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VOLUME AND TEMPERATURE

VVT systems are a popular solution for heating and cooling multiple zone applications in small to medium size buildings. In addition to the central RTU, indoor package unit, or split system, the VVT components include the air source unit controller, bypass system, zone dampers, zone and bypass controllers, space sensors, and necessary safeties to protect the system. VVT controls typically are supplied pre-packaged from the HVAC equipment supplier and are ready to install by the mechanical contractor.

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Figure 4 VVT System Schematic

When only heating or cooling capacity is needed, the system delivers the appropriate amount of air to the zones. Using a timesharing principle, when some zones need heat but others need cooling, the central HVAC unit alternates between providing central cooling and central heating . It satisfies the mode (heating or cooling) with the greatest need then switches to the opposite mode to satisfy the other zones. It does not provide a central source of cooling and heating capacity simultaneously. The complete, pre-engineered control system, comprised of microprocessor-based controls, electronic zone dampers and a bypass system allows the VVT system to deliver the appropriate temperature and quantity of air to the zone from a central heating and cooling source.

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VVT is Variable Volume VVT is called variable volume because it delivers a variable volume of either cold or hot supply air to each zone as load dictates . However, it is important to remember that VVT uses a constant volume packaged unit that must maintain a relatively constant airflow at all times. The VVT system can track the load around the building, maintaining comfort and maintaining efficiency. Modulating zone dampers, with factory or field-installed zone controllers, are used to adjust the volume of air delivered to each zone. Thus, the airflow sent to the individual zones varies over time to meet the changing loads in the zones caused by differences in solar exposure, usage or occupancy, and lighting patterns. In addition, a bypass system is employed so that supply air, which is dampered down at the zones, is mixed with return air to keep the air volume entering the packaged air handler relatively constant.

VVT is Variable Temperature VVT is called variable temperature because the temperature of the air supplied by the central unit varies with time . Each zone gets the same temperature air at any point in time, but the temperature of supply air varies over time. In the heating or cooling mode, as supply air mixes with return air, the temperature entering the packaged unit varies, causing the discharge air temperature to vary. Since most constant volume units have limited steps of capacity, the length of time the compressor is on will vary depending on the number of zones and the volume of airflow required for cooling. The same is true on heating, since limited stages of gas heat or limited steps of electric heat are provided. Therefore, supply air temperatures can vary widely during light loading conditions.

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What is Zoning? When the design of comfort air-conditioning systems for commercial buildings is considered, the issue of temperature control zoning, or simply zoning, is sure to come up . A control zone is a whole building, a group of rooms, a single room or part of a room controlled by its own thermostat or temperature sensor (tstat/sensor). A zoned building is one that has more than one tstat/sensor maintaining its temperature. A zoned system has more than one tstat/sensor controlling the areas it serves . Zoned systems have a Zoned Cooling Medium central source of chilled air System Delivered to Zones Category (all-air systems), chilled water (all-water systems), or chilled refrigerant (direct All-Air Air refrigerant systems), distributed to several zones, each with its own tstat/sensor. AirWater All-Water water systems distribute both Direct chilled air and chilled water Refrigerant Refrigerant from a central source to the zones to do the cooling. VVT Air-Water Air and Water is an all-air zoned system. Other all-air systems include Figure 5 VAY (variable air volume), multizone, double-duct, and Zoned System Categories terminal reheat. Please refer to TDP-103, Concepts of Air Conditioning for a complete discussion on these systems.

Basic System Types VVT (Variable Volume and Temperature) VAV {Variable Air Volume) Multi-Zone Double-Duct Terminal Reheat Chilled Water Fan Coil Unit Ventilators Duct-free split systems (Also called ductless, multisplit, multiplex, and cassette) Conduit Induction Supplementary Air Fan Coil

Zoning is important in maintaining comfort conditions in air-conditioned buildings . However, rather than using a zoned system, like VVT, many commercial buildings are zoned using multiple constant volume, single zone systems, with a rooftop unit serving each zone. Tight construction budgets are the primary reason for this trend. Inadequate to assure individual comfort!

In such cases, the packaged rooftop unit will control the space it serves based on the input it receives from one tstat/sensor located somewhere in the zone. Typically, comfort conditions will prevail in the space where the tstat/sensor is located. However, the remaining areas of the zone may be too hot or cold, resulting in occupant discomfort.

Figure 6 Problems with Constant Volum e, Single Zone Systems

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To overcome uncomfortable conditions, VVT provides additional control zones at minimal additional cost. A tstat/sensor and VVT damper unit added to each room will help to provide comfort to all occupants. This allows the same kind of constant volume, single zone equipment to function on a zoned system without the additional equipment cost normally associated with a zoned system air handler. With WT, a single-zone heating/cooling HVAC unit supplies a zoned system.

Although zoning helps to ensure comfort, it does not come without cost. There is a tradeoff in zoning between customer comfort and installed system cost. More zones provide improved comfort for an additional cost. It is important to evaluate the value of comfort that is added by zoning and the price the customer is willing to pay for it. There is usually a middle ground somewhere between inadequate zoning and ultimate zoning that is acceptable.

Typical Zone Sensor

Figure 7 rl. zoned system, like VVT, eliminates problems.

Types of VVT Jobs Traditionally, VVT has been used on rooftops and split system units under 30 tons capacity. The system is designed to provide a zoning solution in an equipment size range where other zoning options just are not nonnally available. While the VVT system theoretically could be used on larger units, a number of issues in their application should be considered. Items like bypass size and control, or having large numbers of zones, some of which may have unique requirements, will create significant design issues. Larger systems are better done with VA V units or by dividing the space to use multiple smaller tonnage air source units using VVT. The system may also be applied on small units under 5 ton, but the cost is not normally justified. The most common size range for VVT systems is 71/z to 20 tons. The Carrier VVT system can have up to 32 control zones on one air source unit. However, in most applications the number of zones is far smaller. It could be as little as one zone for a specialized zone on a larger non-VVT system, but most often it is 5 to 16 zones on each air source unit. Systems below five zones are not usually economically feasible and multiple air source units may be a better solution. Having a large number of zones can also cause problems since it most likely indicates that a wide range of load conditions must be met with one constant volume unit. This approach greatly increases the chance that one zone will experience excessive temperature fluctuations. Having 6 to 12 zones is usually a good arrangement.

Jobs at 25 Tons or Less The vast majority of VVT applications are designed with HVAC equipment smaller than 25 tons with 3 to 15 ton systems being the most common. When the total installed tonnage for the building does not exceed 25 tons, one packaged rooftop unit is the most common choice.


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Jobs Larger than 25 Tons Since VVT is a time-sharing system, it is best to begin by dividing the building into large areas that have similar load characteristics, with a VVT packaged unit serving each. Then zone each area served by a packaged unit with VVT damper units, one per zone. E In this way, buildings requiring over 0 0 Rental:Area 400 tons of cooling capacity have 95' 0::: ~ been successfully air conditioned us0 RTU1 : RTU2 0 ing VVT. ...... (25 tons>; (25 tons)

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For instance, in this small strip Ii!'.-155' I shopping mall, the 95 ' x 155' rental area on the north side should be conditioned by two rooftops, each with RTU3 RTU4 (25 tons) (25 tons) around 25 tons cooling capacity, be90' cause the area has a peak cooling load N ~ of around 48 tons. The present stock room could use a dedicated rooftop unit; but, since this is rental property Figure 8 and future tenants may not continue Divide large spaces into HVAC unit areas. using this floor space as a stock room, two equally sized rooftops (RTUl and RTU2) will standardize the design and provide future flexibility for tenant rearrangement. On the south, each rental space will require around 25 tons total cooling capacity, so the pattern set on the north side is continued for the south in choosing RTU3 and RTU4.

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• Areas occupied by different tenants • Separate large lighting zones • Areas with special exhaust requirements • Areas with special ventilation or other air quality standards


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Even though VVT can handle a diversity of load patterns, the designer's goal for a larger building is to break it down into areas 25 tons or less, which stabilize VVT system operation by minimizing load differences within each system. Once the building has been divided into areas that can be conditioned by rooftops, indoor package, or split systems; then zone each air source area by using VVT zone damper units. For instance, the south store, supplied by RTU3 , was divided into south and west perimeter zones to track solar variances on those exposures. The display area along the hallway is a separate zone because of its intense lighting load. The core area is divided into 2 zones but could be handled by one. This layout requires 5 zones, with a VVT damper unit in the supply ductwork feeding each zone. Similar zoning work has been done in the north rental area.

Retrofitting Existing Systems with VVT The retrofit of existing HVAC systems with VVT system components has grown in popularity. Generic system compatibility of DDC controls has improved so that the application of VVT upgrade components has become easier than in times past. For instance, the southeast corner of the strip shopping mall was laid out on a modular basis for the smallest rentable retail area, which is 70 ft x 40 ft (2800 sq ft). A 12Yz-ton constant volume, single zone heating/ cooling rooftop unit was installed to satisfy each zone . But, due to perimeter versus core load differences, as well as varying lighting and usage patterns, the new renter of the southeast module, now a separate store, wants to add zoning . This is no problem with the flexibility ofVVT .

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As long as the existing unit is deemed Figure 10 reusable by the engineer, owner or contractor, and is capable of deliver- Southeast Corner, as Originally Designed ing required airflows and capacity to the newly created zones, VVT retrofit is an excellent approach.

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Specify manual, locking balancing damper in each supply branch (typical)

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5-piece metal elbow @diffuser (insulate outside)

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Typical 16-in. round ceiling diffuser Straight flex runs Flex maximum positive and negative static pressure = 2.0 in.wg

Conical sheet metal takeoff for runout connection and installation of balancing damper

Figure 11 Southeast Corner, Constant Volume, Single Zone System

The existing lower pressure class ductwork can be reused for the new VVT layout wherever its size and configuration is correct. In this case, the designer and installer of the original single zone system made the conversion to VVT easy by following good 2-in. pressure class wg duct design practices. To change the system over to VVT, a VVT damper with a zone controller is added to each runout duct. Even though less than 6 zones would work pretty well, the overall cost to modify the duct layout would more than offset any VVT terminal savings. Round 16-in. damper units easily install in each existing branch (runout) duct. Branch balancing dampers are removed, since the VVT damper unit is now in control of supply air quantity to each diffus~r, and there is only one diffuser fed by each damper unit. The supply diffusers are replaced because cone-type diffusers are one of the worst culprits for dumping cold, primary air at reduced airflows. VA V-qualified linear slot diffusers are best but would require radical room air distribution redesign. Instead, high quality, square, multidirectional, anodized aluminum (4-way) louver diffusers are chosen. The louvers are adjustable. Even though these will not hold the cold air up on the ceiling at partial load as well as VA V-qualified linear slots, they should be adequate in this application due to the consistent high lighting load and the level of activity of the people, who are up on their feet, moving around.


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Where a ceiling plenum return is used, a bypass damper with bypass controller must be added in the main supply duct, as in this example. When a ducted return system is used, a bypass damper must be inserted between the existing unit main supply duct and the return main. Be sure to follow the manufacturer' s bypass design recommendations. It is a key element in the stability of VVT system operation. The bypass damper assembly with bypass controller, includes a static pressure sensor. So a static pressure pickup (SPP) is installed in the supply duct just upstream of the first branch duct. Tubing sends the static pressure back to the sensor at the bypass controller. 24" x 24" adjustable louver diffuser (typical)

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Figure 12 Southeast Corner, Retrofitted for VVT System

The majority of VVT projects use new HVAC units and ductwork. This happens when the age of the system being replaced is more than about 10 years and also when the changes to the existing system are too extensive to reuse the existing air system . Caution should be exercised to_ avoid compromising zoning when reusing existing ductwork. Control zone location should be based on building zoning needs and not solely on existing duct runs. It will cost more in the end to fix a zoning problem than to do it right in the first place. An air source unit controller is installed in the rooftop unit and communication bus wiring interconnects all the controllers. Each zone controller is connected to its temperature sensor by sensor w1nng.

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The VVT zone controllers and bypass controller will communicate with other brands of existing heating/cooling equipment when an appropriate fieldinstalled air source unit controller is installed. Carrier' s PremierLink controller will satisfy this need, as shown in this rooftop VVT retrofit. Existing HVAC units and associated ductwork retrofitted with VVT dampers, controllers, and room tstats/ sensors can be an effective and affordable upgrade in comfort for existing systems where only one zone currently exists or for applications in which occupant comfort is inadequate due to the original system zoning design.

Figure 13 Rooftop Retrofitted with VVT Air Source Controller

VVT versus Other Systems One of the most important jobs of the HVAC designer is to pick the right system for the building. Unfortunately, the right system sometimes remains unknown until another system has been installed and is performing inadequately. In many areas of the country, air-conditioning systems must deliver as much satisfaction in the heating mode as in the cooling mode. Consequently, systems are well accepted that can deliver heating and cooling capacity to any zone, on demand, like VVT does when it is applied correctly. This is also one of the strengths of PT AC (packaged terminal air conditioner) systems; standard (non-VVT) water source heat pump systems; duct-free split systems; and chilled water fan coil systems.

• Multiple rooftops, indoor package, or split systems • PTAC (Packaged Terminal Air Conditioners) • WSHP (Water Source Heat Pumps) • Duct-free split systems • Room fan coils (chilled water)

• VAV (Variable Air Volume) VVT cannot deliver heating or cooling simultaneously from the central Figure 14 source to any zone, on demand, when the Zoning System Alternatives to VVT central HVAC unit is in the heating mode, because it relies on a central source for cooling capacity instead of producing it at the zones, as these other systems do.

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VVT Advantages However, VVT offers many advantages over these other systems. PTAC, standard water source heat pumps, duct-free splits, or chilled water room fan coils are typically used when small "repetitive spaces" are required or for room fan coils when a central source of cooling is preferred. These alternatives to VVT have difficulty meeting good indoor air quality standards demanded by ASHRAE Standard 62. Considering the way these systems attempt to meet the filtration and ventilation needs of the building, it makes sense that an all-air system, like VVT, does a far better job. For instance, if chilled water fan coils, water source heat pumps, most duct-free split systems, or PTACs are used for zoning, then the ventilation system must be addressed. Unlike VVT, a dedicated ventilation system, at substantial added expense, will usually be required for these other systems in order to provide ASHRAE Standard 62 verifiable quantities of properly filtered and dehumidified outdoor air.

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Typical duct-free fan coil, high wall mount

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Typical In-Ceiling duct-free fan coil. Horizontal discharge. Multi-Zone split system.

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Poor filtration No ventilation - perimeter Poor ventilation - interior Dedicated ventilation system required to meet ASH RAE 62

Figure 16 Duct-Free DX Split System

While PTAC units offer optional ventilation from the outside, the maximum rated capacity is about 35 cfm and the fan compartment is not sealed from the space. This means that outdoor air can infiltrate directly into the space without being filtered or treated by the unit at all. When indoor pressure exceeds outdoor pressure, the indoor air will exfiltrate out the ventilation opening and ventilation ceases altogether. So the direction and quantity of air moving through the unit's ventilation opening is not verifiable, is highly variable, may bypass the filter and coils, and is subject to wind direction, speed, and building height. These same problems exist when using floor-mounted or ceiling-mounted chilled water fan coils located around the building perimeter with an optional outdoor air wall sleeve and damper. INSIDE

INSIDE

OUTSIDE

OUTSIDE

(f):::::::::::::::::::::i''''\ '''l Floor mount with optional OA wall sleeve

Fan compartment :.. .. .. . .. not sealed from s ace WaterSu

I

._Water Return

Figure 17 Ventilation - PTAC (left) and Room Fan Coil (right)

<1fl6@+1 Commercial HVAC Systems

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In contrast, an all-air system, like VVT, will provide good quality, verifiable ventilation to replenish oxygen and dilute pollutants in the space . In most cases, it can also provide betkr air filtration than these other systems, since the filters on a central packaged unit are more efficient. However, once a dedicated ventilation system is added to the other systems, the filtration can be excellent on those systems, but at substantial added cost. An all-air system, like VVT, can use an outdoor air economizer to provide free or reducedcost cooling when off-peak outdoor air conditions are acceptable . Since the zones hit their peak cooling load at different times, the capacity of the VVT unit need only be as large as the coincident peak or "block" load of the area served; that is, less than the sum of the zone peak cooling loads. By contrast, the installed tonnage of PTAC and water source heat pumps is greater than VVT. It is the sum of the zone peak loads. A load estimating program, like Carrier's E20-II Block Load or HAP (Hourly Analysis Program) will show the difference.

VA V System Comparisons There are several VA V systems that offer comparable zoning to VVT. However, when multiple VVT systems are used to condition a building, VVT often competes with VA V because both are all-air systems providing similar performance benefits in filtration, ventilation, air distribution, aesthetics, and quiet operation in the conditioned space. VA V's traditional advantage is precise, small zone temperature control WT VAV when heating and cooling demands • Higher installed cost occur simultaneously. Both systems . Lower installed cost • A TC contractor usually required provide the energy-saving benefits of · No ATC contractor • :S. 2-in. wg duct design • ~ 3-in. wg duct design an outdoor air economizer and block • WT-qualified dampers and • VAV terminals and diffusers load diversity through the use of a diffusers • VAV HVAC machine single HVAC unit to condition all the • Standard HVAC machine • Retrofit more difficult zones it serves. • Retrofrt easy

• Fan control

VVT has several benefits over · No fan control • Many power connections with VA V. The most significant advantage • Few power connections fan-powered mixing boxes is its lower installed cost in smaller • Minimal fan energy savings • Maximum fan energy savings tonnage sizes . When VA V is de• Poorer part load latent • Better part load latent signed using temperature controls supplied by a manufacturer other than Figure 18 the HVAC equipment manufacturer, VVTversus VAV an ATC (automatic temperature control) contractor will usually be required. When VA Vis designed using on-board, factory-installed controls, it is more likely that the need for an ATC contractor may be minimized or even eliminated, if the installing HVAC contractor is able to install and configure the control system completely. Eliminating the ATC contractor means single-source responsibility for the installing contractor, which streamlines job scheduling and usually reduces installed cost. VVT on the other hand, always comes as a factory-integrated system of product and DDC controls. The configuration is easier and has less variation than for a VA V system, so it is more likely that the cost and scheduling inconveniences of the separate ATC contract can be eliminated.

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Standard, lower pressure class ductwork (~ 2-in. wg) is used on VVT. This is not true on VA V. Since VA V uses no bypass, the supply air system pressure can increase at partial cooling load. This requires a more expensive schedule of reinforcement for rectangular supply ductwork used on main ducts. Spiral round or flat oval mains are an alternative. Duct joints must be tightly sealed to avoid objectionable noise and leakage. Generally speaking, the design and installation of VVT supply duct systt:ms is easier, without specialized methods or materials being required. The return air system design is about the same for VVT and VA V systems . The supply terminals for VA V systems are designed with acoustical attenuation in mind. When the VA V flow control device is located at the diffuser, it is called an integral diffuser terminal. This is a sophisticated diffuser. When the flow control device is in a tt:rminal remote from the diffusers, like a fan-powered mixing box or a single-duct damper box, the box includes the necessary leakage prevention and acoustical treatment.

Integral Diffuser VAV Terminal

Unlike VA V, VVT does not throtFan-Powered VAV Mixing Box tle the fan airflow at partial loads, but Figure 19 rather uses a bypass damper that limits duct pressure so there is limited VAV supply terminals are more complex. potential for a buildup of static pressure in the VVT ductwork. A low pressure VVT damper and diffuser can be selected using low pressure ductwork and less specialized diffusers. However, the diffuser style must avoid dumping cold supply air at reduced airflow in order to avoid discomfort and inaccurate zone temperature sensing. Qualified linear slot diffusers work best. Cone-type round or square diffusers are less capable of holding the air up near the ceiling at partial load during the cooling season. A significant difference between VA V and VVT systems is the central HVAC equipment. VVT is able to use a standard, constant volume, single zone unit because the air volume never goes below the manufacturer's minimum recommended limit. On the other hand, VA V requires a variable volume qualified unit, which is substantially more expensive than a standard unit because it must have fan control (typically a VFD), controls to maintain discharge air temperature, and more steps of part load capacity. Therefore, VVT systems can utilize existing constant volume air sources, while VA V systems cannot. The VA V retrofit requires such extensive modifications to the central machinery that it is usually impractical to attempt the change. The central equipment is usually replaced when going to VA V. Figure 20 VAV air handlers have added components.

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Without VA V modifications to the central unit, if applied on a VA V system, the central unit can experience unstable operating conditions and frosting of the evaporator coil, causing nuisance tripouts on equipment safety limits as well as liquid refrigerant floodback, which will hann or destroy the compressor. In addition, the VA V fan will require a VFD (variable frequency drive), inlet guide vanes, or other static pressure control device to optimize fan energy savings and limit static pressure buildup at pa1tial cooling loads. VVT systems require no fan control device since it uses a bypass damper. When comparing VVT with a VA V system that uses fan-powered mixing boxes, VVT requires less line voltage electrical power wiring and fewer points of connection. The VVT electrical distribution system will cost less to design and install. Each VA V fan-powered mixing box has a small electric motor that requires line voltage electrical power wiring for each zone. VVT damper units require only 24 volts for the damper motor. Both systems require some type of control connection at each zone. Fan-powered mixing boxes may be used for VVT. When this is done, the electrical distribution cost benefit over VA V no longer exists. The same electrical distribution system cost savings exists when comparing VVT to PTAC, water source heat pump, duct-free split, and chilled water room fan coil systems, since each of these systems have a line voltage motor in each zone. The PTAC also contains the compressor, so it has the greatest requirement for electrical power distribution to each zone of all systems considered so far. However, the PTAC unit includes controls, so there is no additional cost for control wiring as on the rest of the systems. Another main benefit VA V has over VVT is its part load energy efficiency. This comes from fan energy savings at partial load and no mixing of hot and cold airstreams with the bypass, which is something VVT cannot achieve. Generally speaking, the bigger the job, the higher the monthly power bill and the more important fan energy becomes to the owner.

VVT versus Multiple Units VVT is a cost-effective solution to zoning, especially when applied on commercial applications that require 5 or more zones . An alternative, against which VVT is measured, is zoning the building using constant volume, single zone equipment, one per zone, typically rooftops. Multiple packaged units used in this way are typically more cost-effective when providing 4 or less zones of control in the buildmg. Above four zones, VVT reduces installation cost by minimizing the number of HVAC units required and

WT is the right choice when : • 5 or more zones may be necessary • Centralizing and reducing unit maintenance is desirable

OR

-

Filter changes Belt adjustments Economizer adjustments Yearly maintenance costs

• Reducing installed cost by minimizing: -

HVAC units Power supplies and disconnects Duct system RiQging, piping and wiring Roof penetrations Roof curbs

Figure 21 VVT versus Mu ltiple Constant Volume, Single Zone Un its

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VARIABLE VOLUME AND TEMPERATURE the expenses associated with providing multiple roof penetrations and curbs, rigging, electrical power connections, electrical disconnects, control wiring, piping, and ducting. VVT also keeps the heating and cooling equipment maintenance centralized and at a point outside the occupied spaces of the building. This means that the number of maintenance points is reduced and that scheduling of maintenance or repair does not disrupt the occupants. Both of these features are attractive to customers who are thinking in terms of the life cycle cost of the system and the indoor environmental quality of the building spaces. Another advantage of VVT is in applications where multiple, small zones of control are required and separate units are not available in sizes small enough for each zone. Combining the small zones creates a total capacity requirement in a size range that can be met by a single central unit with VVT control.

Zoning the Building for VVT In defining zones, one should be logical and prudent in their selection since increasing the number of zones adds to the installed system cost. Hence, the designer should be careful and not give in to the urge to give every room a sensor. Building zones are typically required because of differences in the following factors from one area of the building to another:

~~M

100'

II

iw£1V 0

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I

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• Space usage

60' 2

• Glass exposure (ft and %wall area) • Glass orientation (e.g. N, E, S, W, etc.)

~® -~-~-~--~-----

• Occupant schedule • People density • Lighting control zone • Lighting level • Perimeter versus core exposure

® - -- - ----Zone Boundary ® Zone Number Figure 22 60 'x 100 'A1anufacturing Office Floor Plan with Partitions, Dimensions, and Zone Numbers

• Roof exposure or none • Occupant responsibility (authority in building management) • Wall exposure (area and orientation) • Tenant variations (schedule, preference, billing) • Special ventilation needs

./

• Special exhaust needs • Special air cleanliness requirements

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As you can see, there are many factors that could drive the designer to make an area a separate zone of control. So, to simplify the zoning decision, the designer must identify the differences that exist and prioritize the importance of them. As zoning decisions are made by the designer, the floor plan is an indispensable tool. The zone boundaries should be drawn on the floor plan as decisions are made. In order to economize on system cost, similar rooms should be grouped together to form a single control zone.

Group together areas with a similar solar exposure. First, separate the core from the perimeter areas. Then separate perimeter areas based on orientation. Along the perimeter of the building, solar load usually constitutes about 45 percent of the peak room sensible cooling load for a standard, single-story office. Since this load is most significant, it is important to group exposures with differPercents of zone room ent peak times separately. Also be sensisensible heat tive to shadow lines along a single expoEach mark = 1% sure, which are cast by neighboring buildings, covered walkways, or trees. A shadow makes east, south or westexposed glass and walls perform as if it were facing north. Figure 23

The manufacturing office demonstrates typical peak cooling load times for various exposures. Coupled with the large impact of solar load, the peak times show why the east, southeast comer, south, southwest comer, and west office spaces were placed on separate control zones. An additional consideration was that the southeast and southwest comer offices an~ occupied by company management personnel and that the building is owner-occupied, with a higher expectation for comfort and a greater willingness to pay for it than would be the case for a speculative (rental) building of the same design. Typical Perimeter Zone Sensible Cooling Load Components

Transmission, the conductive heat transfer across all external barriers caused by the difference in temperature from outside to inside, is significant for perimeter zones. However, it is not a consideration in zoning because the outside air tern perature and inside air temperature are uniform all around the building. Only differences in loads and load patterns stimulate a zoning consideration. Load calculation methods often combine

·---- - - - - - -1- 1 0 0 ' - - - - - - - - - - - -

M

Engineering ul, 4 p.m. 6 people

lw 0

General Office (core) Jul, 4 p.m. 30 people

Conference Room Jul , 9a.m. 12 people

© 60'

~-L-~~--I ____ _ ® I (perimete~ Chief Engr.

Sep, 3 p.m. 2 people

I .

General Oct, 2 p. . 1O people

@

.

Manager Sep, 10 a.m. 2 people

@

--------Zone Boundary • Zone Number

Figure 24

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Zones, Owner-Occupied, Fixed Partitions


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the effects of solar on walls and roofs with the transmission. Wall and roof solar loads are important in zoning decisions since the mass and exposure will impact the load. The perimeter zone pie chart (shown in Fig. 23) also shows that lights and equipment (computer) are a significant part of the zone load picture. It constitutes about 25 percent of the sensible cooling load in sunlit perimeter spaces. If a person in one office turns off the lights and computer and leaves, but the person in the adjacent office on the same exposure continues working, the temperature difference between the two may become unacceptable if they are in the same control zone because there is only one tstat/sensor to respond to the two circumstances. Differences greater than 3-5° Fare usually considered too uncomfortable. In the cooling mode, the difference in dry bulb temperature between the air coming out the diffuser and the room temperature is 15-20° F, say 16.5° F (with the supply diffuser outlet temperature at 58.5° F and the room temperature at 75° F). This 58.5° F cold Roof air warms to room temperature as it ZD absorbs the sensible loads entering the Balancing Damper room. So if lights and equipment . , zc CommunTcaiion : ~ (computer) constitute about 25 perT0 = 58.5° F Bus ; cent of the room sensible heat added • ~~~--....,.-~~-=< to the space, then about 25 percent of l /so'-. RG the warming of the supply air can be Flow reduced swl Flow reduced At from room sensible : Gt from room attributed to lights and equipment. ([)sensible heat = 16.5° F heat "' 16.5 + 4° F =20.s• F With a difference of 16.5° F at the ZS ITRM= 75° F I ITRM= 79° FI supply diffuser, this amounts to about No lights or computer Lights and computer on 4° F (16.5° F x 25%). If that load is @ @ present in one space in the zone but absent from the other, then under the Figure 25 these circumstances, the difference in Zone Temperature Difference when Zoning is Compromised temperature between the 2 spaces will be about 4° F . This 4° F difference is somewhat reduced by the mass of air in the room, the barriers around the room, and the time it takes to reach equilibrium, however it may still be considered unacceptable. Based on the pie chart and the zoning work done so far, it makes sense that core areas and north-facing areas should not be plagued by variations in solar loads. Therefore, their load patterns should be more similar than pe1imeter areas . However, the pie chart for a typical core area with roof shows a potential problem with this reasoning. Since solar and transmission Percents of zone room loads make up less of the load in the sensible heat core than in the perimeter areas, the people, lighting, and equipment loads Each mark = 1% have a more significant impact in the core than the perimeter areas. Therefore , if the occupancy, and especially the lighting patterns, are the same throughout the core, then the core can Figure 26 be treated as one zone of control. Typical Core Zone Sensible Cooling Load Components (with roof) However, if floor-to-ceiling partitions

21


are installed, and lighting and occupancy patterns vary within the core, then zoning will be demanded even more in the core than in the perimeter areas of the building. If partitions and light switches are added to the core in the future, VVT is a system that has the flexibility to add zones of control as needed, with minimal system changes. A core area on a floor without roof overhead is even more sensitive to occupancy and lighting variations because the lights, equipment and people comprise the entire room sensible cooling load. In those cases, lighting may contribute up to about 80 percent of the room sensible cooling load. The impact of people and lights required the use of occupancy zoning in our example for zone 1, the conference room. Even though the peak time caused by exposure is identical to Office 2, the potential load variation caused by people and lights in the conference room prevents placing these two areas on the same control zone. The magnitude of the load caused by occupancy and lighting can Solar be seen from the pie chart shown. (30%) In addition to people and lighting variances, the special, intermittent ventilation need for a conference room full of people also demands a separate zone of control. VVT offers features that will accommodate the need for increased ventilation to this important area of the building.

Percents of zone room sensible heat Lights

Each mark = 1%

(25%)

Figure 27 Conference Room Sensible Cooling Load Components

Basic Sequence of Operation VVT is demand-oriented and responds to individual zone loads (see Figure 24). The load conditions in the space control the equipment capacity through a sharing of consolidated information and modes of operation between system components over a communications network. This system coordination, or "linkage," is intelligence that resides at the linkage coordinator and within the air source controller, located at the rooftop unit. The linkage coordinator, chosen at the time of system configuration, is located at a zone damper. The linkage coordinator is responsible for operation of the VVT system and sends out one set of inputs to the air source controller that is responsible for ef1icient and reliable operation of the air source equipment. Before we discuss actual operational sequences, we need to define two important concepts: linkage and pressure dependent/pressure independent.

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Linkage Linkage simply refers to the process through which data is exchanged between the unit controllers at the air terminals, bypass controller, and the air source controller. The process "links" the VVT damper terminals, bypass damper and air source to form a coordinated system. Linkage allows the air source to operate efficiently and reliably while responding to and satisfying the changing conditions in the zones. Linkage also allows the zone terminals to respond properly to changes in the air source, so the feedback is mutual. A linkage coordinator, located at a zone VVT damper, coordinates the Zone Controller Air Source Controller flow of data between the air Zone space temperature Air source linkage table source unit controller and Occupied heat and cool set points Air source update flag VVT zone unit controllers. Unoccupied heat and cool set points Operating mode Occupancy status Supply air temperature The linkage coordinator is Bypass Damper position Start bias one of the zone controllers Controller Demand selected when the system is Damper size AS linkage tables co, Level (optional) configured. It will poll the Coordinator zone's address & bus %RH (optional) requirements of all the other Computed occupied & unoccupied heal/cool set points zone controllers and send the air source controller one set of integrated requirements representative of weighted requirements of all the zones. RED= INPUTS BLUE =OUTPUTS The air source controller can Figure 28 then respond as if all the zones were one weighted av- Linkage - Flow chart showing the information passed back and forth between the controllers used on a VVT system. erage zone. The information exchanged between the linkage coordinator, bypass controller, and air source controller flows both ways. As you can see, a substantial amount of information is exchanged. In fact, this electronic dialogue between the VVT unit controllers is the means by which the system is controlled in a comfortable, coordinated, energy-efficient manner. For further information about linkage, please refer to the, Controls, Level 2: DDC Networking TDP .

Pressure Dependent (PD) versus Pressure Independent (PI) The zone controller, which is used at each VVT zone damper terminal, has traditionally been a pressure dependent device. However, by choosing an optional zone controller for selected zone dampers, those zones may be made pressure independent. The term pressure dependent means that as the static pressure in the supply duct changes, the airflow volume through a given damper opening also changes . Therefore, the zone airflow is dependent upon the zone ' s supply duct static pressure at any damper position. With a pressure dependent control strategy, zone damper position is modulated to maintain a zone temperature set point, without regard for duct static pressure. The speed of the control loop response is assumed to be quick enough that duct static pressure variances will not disrupt the space temperature in any significant way. This usually proves sufficient for jobs that do not have a specific airflow requirement.

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Pressure independent means that the airflow volume at the zone remains constant even though the supply duct static pressure changes. With a pressure independent control strategy, zone damper position is modulated to maintain a desired airflow set point based on maintaining comfort in the conditioned space. Where zone PRESSURE DEPENDENT airflow must be maintained constantly at some level for the sake of venti- I Zone Sensor I , \_ Zone Zone _/ , I Zone Sensor I Controller L...............
~:!><1

• Controls damper position based on zone demand

• Controls damper position based on zone demand

• Corrects for cfm variance caused by static pressure variance by responding to zone temperature variance

• Anticipates and compensates for the cfm variance caused by static pressure variance • Least typical

• Most typical

• Higher installed cost

• Lower installed cost

• elm constant with varying duct static pressure

• cfm varies with duct static pressure

• Pressure sensor required at each zone

• No extra sensors

Figure 29

Call for Heat/Cool and Equipment Mode

VVT Zone Controller - Pressure Dependent versus Pressure Independent

In order to understand how the DDC controls manage the VVT system, what follows describes one manufacturer's approach to various situations that require a change of mode. A VVT system' s mode of operation can be heating, cooling, or ventilation. The mode is determined based upon whether there is a call for heating capacity (zone 4), cooling capacity (zone 2) or no call at all (zone 3). The demand a zone has for capacity is de------ --- 4/\ zc ' termined automatically, ZD ZD ZD ZH once per minute on a . W'--~+---.--~.~~W---~+---.--~.~~W---~-1--r-'11 RG continuing basis, by the i /so'-.. RG l /so'-.. RG l /so'-.. RG SW :l SW : : SW i: linkage coordinator as itSP = 75° F SP = 75° F SP = 72° F gathers demand infor© ZT = 75.5° F © ZT = 75° F © ZT = 70.5° F ZS 0 = 0.5 clg ZS 0 = 0.0 ZS D = 1.5 htg mation from itself and all the other zone controllers. Each controller calculates its cooling or SP = Zone temperature set point ZT = Actual zone temperature heating demand as the D = Demand (htg or clg) difference between the mode set point and the Figure 30 actual zone space ternVVT Zon e Capacity Demand perature.

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In order to normalize a system of combined pressure dependent and pressure independent zone controllers, a maximum airflow for pressure dependent zones will be calculated, allowing the linkage coordinator to collect the same data for pressure dependent and independent zones. The linkage coordinator calculates the total average heating and total average cooling demand automatically, allowing the airflows to be derived from those formulas and the total average cooling and heating demands to be displayed at the system pilot, in the degree difference from set point. The average demand is a weighted average based on the airflow delivered to a zone. In other words, a 1000 cfin zone will count twice as much in the demand average as one that re ceives only 500 cfm. If no mode is currently active, the VVT linkage coordinator will determine the mode by first comparing the average heating demand and the average cooling demand. If only one demand is greater than or equal to the minimum demand configured to start a mode of capacity, the system will start that mode . If both average demands are greater than or equal to the configured minimum average demand required to start a mode, the mode with the greater demand will be selected. If both heating and cooling average demand are exactly the same, then the mode with the greatest individual zone demand will determine the starting system mode.

System Changeover Once a mode is started (e.g . cooling), a timer is started to monitor the elapsed time of the operating mode. A mode will end when the average demand for that mode falls below the fieldconfigured minimum average demand setting necessary to activate that mode. If a system mode is currently active (e.g. cooling), and the average demand for the opposite mode (e .g. heating) becomes greater than that for the current mode (cooling), the system will switch to the opposite mode (heating). However, this can only happen after the elapsed time of the current mode (cooling) exceeds the field-configured minimum elapsed timer value. Average Heating Start Demand Satisfied NO

Average Cooling Start Demand Satisfied

System Mode

YES

YES YES

NO

Cooling Heating *Reselect

YES

______*-~-v_~~9_g~_ H~_"!t _Q ~_rn9n_~ -~- A\1~!_'=!9~ _c;:_~ol _Q_~n:i§l_!Jd_____ _--------- ---_lj_~~~! ~~- -----------_____ -~~y_~~9_g~-H~_C!tQ~_rn9_n_~ -~ A\l~_r_"!9~_c;:_~
When these conditions are met, information will be sent to the air source controller that will end the current mode (cooling) once the supply air temperature comes within the ventilation or neutral temperature range. At that point, the other mode (heating) is started if the average heating demand exceeds the minimum required to start heating. The purpose for the elapsed timer check is to prevent needless short cycling. Once a mode is ended (e.g . cooling), a built-in algorithm prevents the original mode (cooling) from restarting unless there is inadequate demand to start the new mode (heating).

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VAR IABLE VO LUME AND TEMPERATURE

Selecting Zone Priority - Reference Zone Once the required mode has been selected (e.g. cooling), the linkage coordinator identifies the reference zone as the zone with the greatest need for that mode (See Figure 30). The magnitude of need for that mode (cooling) is determined by the absolute demand. In the example shown, if the mode is heating and zone 1 (not shown) has a heating demand of 1.0, then zone 4 is the reference zone. The reference zone is a moving target over time because the reference zone is redefined each time the linkage coordinator gathers data from the zone controllers, which is once per minute. This ensures that the equipment can meet the most stringent system capacity needs. To allow the equipment to operate in an efficient and timely fashion, the reference zone ' s space temperature and set points (heat/cool) are sent to the air source controller by the linkage coordinator.

Fan Sequence of Operation Fan operation is a vital element in achieving adequate air cleanliness and distribution. Occupancy status is important in determining fan operation. When people are present consistent air distribution and IAQ are required; but when the Occupancy Fan Operation Fan Configuration space is unoccupied, temStatus/Schedule perature is the diiving priorAny zo ne is Continuous On ity with air motion and IAQ occupied of lesser concern. OccuAll zones Cycles with call fo r pancy status is determined Continu ous heating/cooling un occupied by the occupancy start/stop Cycles with ca ll for settings made at each zone Doesn't matter Intermittent heating/cooling controller or made globally at the linkage coordinator when the system is eonfig- Figure 32 Fan Sequence of Operation ured. The system occupancy function, at the linkage coordinator, will send a summary of the occupancy status of all zones to the air source unit controller. If any zone is occupied, the system will tell the air source unit controller that it should be in the occupied mode. With the fan configured for "continuous" operation, the fan will run continuously and ventilation air dampers will open. Once all zones become unoccupied, the ventilation dampers will close and the fan will cycle on only when there is a call for heating or cooling. With the fan configured for "intermittent" operation, the fan will cycle on only when there is a call for heating or cooling, regardless of whether the air source unit controller is in the occupied or unoccupied mode of operation. The ventilation dampers .will cycle with the fan . This is normally not allowed on commercial buildings .

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VVT Air Distribution System Design As with any all-air system, the design of the air conduits that convey the air from the central source to the diffusers in the zones and then back to the central source through the return system is very important. While VVT is an easy air system to design, it is imperative to use good industry-approved practice for 2-in. wg ductwork systems to achieve optimum comfo1i and efficiency. Here are some specific design guidelines. See Figure 11 for some aspects to concentrate on in order to achieve a good design. • Main ducts at 1200 fpm maximum

In order to avoid higher static pressure and noise problems, it is important that VVT ductwork not be undersized. Size main duct sections for a velocity not to exceed about 1200 fpm (feet per minute). Branch ductwork, where zone dampers are installed, should not exceed 1000 fpm. The bypass duct and damper should also not exceed 1000 fpm.

• Branch ducts at 1000 fpm maximum • Use equal friction or static regain method • Target friction rate = 0.08 in. wg I 100' EL • Aerodynamic fittings - especially near unit • Bypass duct :s; 1000 fpm @ unit cfm minus (smallest zone + ventilation cfm)

Since the dampers continually rebalance Figure 33 airflow to meet zone load needs, the rules for VVT Duct Sizing Tips sizing VVT supply duct sections are not as critical as those for a constant volume system. Either the equal friction or static regain methods are acceptable, or a combination of the two (modified equal friction method). Most designers prefer the equal friction method for the capacity range of VVT systems. When the equal friction method is used, the initial section of supply and return duct right near the air source should be sized using an assigned velocity, while subsequent sections should be sized for about 0.08 in. wg friction rate (e.g. 0.08 in. wg friction loss per 100 feet of equivalent duct length). Runouts that connect to zone dampers should be sized at the damper inlet size. Use the most aerodynamic duct layout possible at the connection to the air source. Use rectangular, radius elbows instead of a supply plenum whenever possible. The radius elbows for supply should have single thickness turning vanes. Rectangular mitered elbows with double thickness turning vanes are the next best option for supply ductwork. Use acoustically lined, rectangular mitered elbows without turning vanes at the return connection to avoid line-of1) sight fan noise transmission from the unit back through the Roof curb return duct to the conditioned space. When a supply or return plenum must be used, minimize turbulence, noise and dynamic losses by using coniFlexible duct connector to isolate equipment cal tap-in connections at the vibration from ductwork plenum for the branch or Rectangular mitered elbows runout ductwork. Be sure to or plenum; no turning vanes Return install flexible connectors at Figure 34 Designing Aerodynamic Ductwork at Unit

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both the supply and return connections to the air source. This will avoid the transmission of equipment vibration and noise through the sheet metal ductwork into the occupied spaces. Some designers choose to use diversity when sizing VVT supply ductwork. This is an optional strategy, but can reduce the size and cost of the duct sections nearest the air source . Since VVT reduces the supPeak RSH ply airflow that is not needed at Zone cfm Zone Peak Time Cool Load partial cooling load sections of 681 1 Jul, 2 p.m. 12,360 the main supply duct, feeding 5,760 317 2 Jul, 2 p.m. multiple exposures and the initial 509 3 Sep, 10 a.m. 9,240 return main closest to the air source may be downsized by us1,461 Oct, 2 p.m. 26,250 4 ing a diversity multiplier. The 8,280 456 5 Sep, 3 p.m. initial supply and return main 1,084 Jul , 4 p.m. 19,680 6 duct sections up to the first major 37 ,080 2,043 7 Jul, 4 p.m. branch can be sized to ret1ect the 118,650 6,551 L: Peaks peak block load cooling diversity 5,600 Aug, 4 p.m. 101 ,160 Block because even at peak block cooling load (about 4 p.m., August), not all the zones are peaking. The Figure 35 west, north and core zones are at Manufacturing Office Peak Loads and cfm - Newark, New Jersey or near peak load, but the east, southeast comer, south and southwest comer zones are off peak. The diversity factor is the block room sensible cooling load or cfin divided by the sum of the peak zone room sensible heat gains or cfms. For the manufacturing office, the sum of the peak zone cfm for all zones is 6551. But the estimated, diwrsified block cfi11 for the rooftop unit is only 5600 cfm, based on the load estimate or, better yet, the equipment selection. This gives a block load diversity multiplier of 0.85 , which is 85 percent (5600 I 6551 x I 00). Therefore, the first supply and return duct section can be sized for 5600 cfm rather than for the sum of all zones at peak load, 6551 cfm. Subsequent sections of ductw-ork use no diversity. They are sized using the sum of the peak cfm for the zones they feed. As always, summarize the duct section cfm from the zones back to the central unit, then size the duct sections from the unit out to the zones it is serving. Trunk ductwork is the main ductwork that is connected to the air source at one end and the branch ductwork or runout ductwork at the other end. The trunk duct should be laid out: • to coordinate with the best terminal and supply diffuser locations • to make provision for as smooth and aerodynamic connection to the air source as possible • so that the connection lengths for the run-outs are as short as possible • in straight lines • in a configuration that simplifies the entire layout • in a symmetrical arrangement for economy and to facilitate air balancing • to fit the clearances of the space where it runs (be sure to count duct insulation thickness in allowing clearance)

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Branch ducts connect the main ductwork to the runouts . Runout ducts then connect to the supply diffusers and, when a ducted return system is used, the return air grilles. Some systems need no branch ductwork, so they only have main and runout ducts. Branch and runout ducts should be arranged so that (see Figure 2): • when a zone has multiple diffusers, there is a balancing damper for each diffuser in the runout duct near where it meets the plenum at the zone damper unit or the branch duct which the zone damper feeds (Figure 2, south zone) . • multiple diffusers in the same zone can be controlled from a single VVT zone controller (Figure 2, core zone). • the main connection from the VVT zone damper is located in the center of the branch duct that feeds the diffusers. This is economical and simplifies air balancing (Figure 2, south zone). Wherever possible, use sheet metal ductwork with an external insulating sleeve and vapor barrier instead of pre-insulated, flexible, wire-helix ductwork. Flex duct attenuates sound and speeds installation but, under the best cases, has roughly three times the pressure drop as the same diameter sheet metal duct and it has a higher material expense. Where flex ducts are used (see Figures 2 and 12): • upsize it appropriately. An 8in. flex carries what a 6-in. sheet metal duct carries . • avoid sags and bends

Avoid the use of extractors. They add costs and create unnecessary turbulence to the system. Use a conical cake off instead

Figure 36 Avoid Flex Duct Abuses

• keep to a maximum length of about 6 ft (unless local code is more restrictive) • do not hang flex duct by wire, narrow metal bands or zip ties . They will crush the flex over time and constrict its opening as much as 50 percent.

Comidf:r afan-p(l)wepet/J mixing bf!JiX in pJace of the J?VT damper

• hang flex using a cradle made from halfsection of a snap-lock round duct, about 12 in. long, supported at its comers by . . wire suspens10n . • install balancing dampers in a sheet metal sleeve back at the zoning damper or branch duct, not in the flex duct • never make an elbow above a diffuser by turning a 90° bend in the flex duct. This will destroy the capacity and air diffusion pattern of any diffuser. Use a sheet metal elbow instead and externally wrap it.

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29

Turn to the Experts.


Sealing VVT Ducts Air leakage from the ductwork is bad on any job. When using a VVT system, maintain the same standards of high quality ductwork construction that you would on any good lower pressure class job. For good duct design practice, follow SMACNA (Sheet Metal and Air Conditioning Contractor' s National Association) duct construction standards, as published in their handbook, "HVAC Duct System Design." The standards for sealing air systems against leakage, found in ASHRAE Standard 90.1, follow the SMACNA/ASHRAE 90.1 VVT Duct Sealing Standards(< 2 in. wg) SMACNA standards. Primary Sealant Duct Location

Sheet Metal Type

Transverse Since VVT systems operate Better than below 2 in. wg static pressure, Round Snap-Lock pressure-sensitive (duct) tape they are considered "Class B Seal Unconditioned Better than Level" when run through an un- (Class B) Round or flat pressure-sensitive Includes ceiling pipe oval spiral conditioned space, such as a ceil- vaults with (duct) tape Rectangular Better than ing space when a system uses Ducted Return snap-lock or pressure-sensitive ducted return, or through any Pittsburgh Seams (duct) tape other unconditioned area of the Conditioned or in Pressure-sensitive plenum All building. They are considered ceiling (duct) tape return (Class C) "Class C Seal Level" when run through an air-conditioned space Figure 37 or a ceiling space when the sysSMACNA Duct Sealing Guidelines (.:5' 2 in. wg) tem uses a ceiling plenum return.

Longitudinal Better than pressure-sensitive (duct) tape None required Better than pressure-sensitive (d uct) tape None required

For Seal Class C, pressure-sensitive duct tape is suitable on transverse seams (i.e. at duct section joints). Longitudinal seams need not be scaled. However, Seal Class B requires that both transverse and longitudinal seams be sealed with something better than pressure-sensitive tape as the primary sealant. Many approved sealing systems are available for this purpose. Therefore, to comply with ASHRAE Standard 90 .1 duct construction standards for Seal Class B situations, round sheet metal "Snap-lock" branches and runouts must be sealed along their entire length with something better than traditional duct tape. This is also true for the Pittsburgh or snap-lock seams on rectangular main ductwork. In these circumstances, the use of round or flat oval spiral pipe may prove more cost-effective because, as the guideline says, "Spiral-lock seams need not be sealed." Spiral pipe will save substantial duct construction labor over conventional lower pressure methods .

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From a duct design viewpoint, enforcement of ASHRAE Standard 90. l may also encourage the use of ceiling plenum returns because of the less stringent duct sealing standard than for ducted return systems. However, this should not drive the air system design decision. Ceiling plenum returns substantially reduce the latent (dehumidifying) cooling capacity of the packaged equipment typically used on VVT jobs, because the return air temperature is significantly warmer than on a comparable ducted return system. Since packaged rooftop units, indoor package equipment, and split systems using packaged air handlers can have difficulty delivering ample latent cooling capacity at partial cooling loads in humid climates, a ducted return is the best policy. An alternative is to equip the air source with factory accessories that enhance latent capacity. Consult TDP-631, Rooftops, Level 1, Constant Volume, for a discussion of these accessories .

Dampers A wide range of both round and rectangular dampt:r sizes may be used for the VVT system. Both aerodynamically and economically, it is best to choose the same size and shape damper as the duct in which it is installed. Minimizing the number of dampers also cuts cost, since it usually costs less to install one larger zone damper than two smaller ones. HowZone Duct Temperature ever, the zoning choice should not be Sensor compromised just to minimize the zone damper cost. The recommended maximum damper velocity is 1000 fpm, just like Inlet Size Maximum (in.) cfm the branch ducts in which they reside. 200 6 I Round and rectangular dampers 8 350 should be selected based on required 550 10 peak load airflow (cfm) to the zone. --- 12 800 14 1100 Be sure the cfm falls between the 16 1400 --mmnnum and maximum ratings specified by the manufacturer. lf Figure 38 space restrictions do not allow the use of a single damper for a large zone, up VVT Zone Dampers to four additional dampers can be wired together to provide the required airflow.

Rectangular dampers available in some larger sizes

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31


Round Dampers Round dampers are preferred when round ductwork will fit into the ceiling space. The damper blade is not round, but elliptical, with a gasketed edge for good shut off. The blade rotates in a modulating manner from fully open to fully closed and can stop at any point in between. As the size of the damper, and hence its airflow, gets larger, the torque required to close a single large blade increases. Therefore , use a high-torque actuator when indicated by the manufacturer on larger sizes or where the static pressure Figure 39 drop or turbulence is high. Looking into a Round VVT Damper

Rectangular Dampers Whenever rectangular main supply ducts are used at the air source connection, a rectangular bypass damper makes the most sense. This is also true anywhere a damper is installed in rectangular branch ductwork. Like the round VVT damper terminal, rectangular damper units use single blade construction with a gasketed edge for good closure. With a rectangular damper, the requirement and cost to interlock multiple dampers is eliminated since one larger rectangular damper can take the place of several round dampers and still fit within the height restrictions of the job.

Bypass System Layout The bypass system is a very im portant part of the VVT system because it serves two important functions. First, the bypass system keeps supply duct pressure down at partial load by maintaining static pressure set points. Without the bypass, the restriction of closing zone dampers would cause duct static pressure to climb, adding stress to all supply duct components . Accompanying velocity and noise generated at partially open dampers would cause problems in the conditioned space.

WT application utilizes a constant volume HVAC unit in a VAV like strategy.

Supply Air

CEILING PLENUM RETURN

Zone Dampers reposition constantly to maintain occupant comfort. The HVAC unit puts out a constant airflow, so static pressure changes with demand.

)=====;

(::.._,--_--_:"'-""'-- --"""'-

-"

Static Pressure Pickup

Ceiling-----------------

Bypass controls static pressure to avoid over-pressurization

Fig ure 40 Plenum Return Bypass

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32


Secondly, the bypass setup ensures that the minimum, safe airflow will be maintained at the air source at all times. This pennits VVT systems to use standard, constant volume, single zone HVAC equipment to provide zone level comfort. Without tht: bypass, the restriction of zone dampers that an.: closing at partial load would reduce the airflow through the unit below the manufacturer' s safe lower limit, which is around 300 to 350 cfin per ton of total cooling capacity. This causes equipment problems such as freezing condensation on the outside of the evaporator coil and liquid refrigerant floodback, which slugs and eventually destroys the compressor. Bypass is also important in the heating mode (e.g. a gas-fired heat exchanger and electric heat require that minimum airflow be maintained over the heat exchanger or coil at all times to prevent tripping on high limits). VVT bypass satisfies this airflow requirement for gas or electric heaters in the central unit.

Bypass Components The bypass system consists of: • A bypass damper or dampers sized to handle 75 percent of the total design airflow • A bypass controller • A damper actuator for each bypass damper • A duct static pressure pickup, which taps into the supply duct • Static pressure tubing from the pickup to the bypass controller

Functionality The bypass controller modulates the bypass damper open or closed, from zero to full flow, allowing varying amounts of air to pass directly from the supply system to the return system without circulating through the building. When the bypass is closed, all the air passes through the HVAC heating/cooling unit, the supply ductwork and diffusers, and the return air system back to the unit. As tht: bypass opens more and more, air bypasses the supply ductwork system downstream of the bypass damper but still flows through the equipment. There are an almost infinite number of intermediate bypass damper positions where some of the air goes through the supply system and the rest by- Figure 41 passes it, going directly into the return r1 Bypass Damper with Controllerlrl. ctuator system. The vast majority of the time, the bypass damper is in one of these intermediate positions, because it is only at peak load times that the bypass is fully closed, which is less than 1 percent of the hours in a year.

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The bypass damper modulates in order to maintain a configured supply duct static pressure, which is sampled by the static pressure pickup in the main supply duct (Figures 4, 42, and 43). The air temperature is measured by a temperature sensor, separately supplied and mounted in the air source discharge duct. During changeover, the system mode reported to the zones will not change from the old mode to the new mode until the system supply air temperature reflects that the air source has made the change from one mode to the other. For instance, if the system is changing from cooling to heating, the zones will continue in the cooling mode, responding to a cooling demand, until the system supply air temperature increases to indicate that the air source is now providing warm air. This prevents uncomfortably cool air from going to zones that require heating during the time it takes the unit to switch over to heating and raise the temperature the air source is supplying. The setting, or configuration, of the bypass system operating set points is done by using the system pilot, which is also used to configure all settings for the system when the job is commissioned. The intelligence for controlling the bypass system is at the bypass controller, mounted on the bypass damper assembly. New designs of VVT systems do not require a dedicated bypass controller thermostat, as in the past.

Layout Particular attention should be paid to the bypass design. It can be the source of operating problems when neglected . The good news is that the design of the bypass is simple when some basic guidelines are observed. The bypass layout is important, especially its location relative to the unit, discharge air temperature sensor, bypass pressure pickup, return duct, and return grilles. The layout in Figure 42 shows a proper design for the manufacturing office when a ceiling plenum return is used. The bypass system can also be designed for a ducted return system (Figure 43). For systems using a ceiling plenum return, be sure to mount and support the bypass damper firmly. Do not rely on the sheet metal of the main supply duct alone to support it. Point the bypass discharge away from the return inlet. The goal is to have the bypassed air mixed with the ceiling plenum (return) air to the greatest possible extent before entering the return duct. Many designers feel that a ceiling plenum return does the mixing job better than a ducted return. Good mixing stabilizes equipment operation and avoids nuisance trips on safety limits, particularly a_t low load. However, ceiling plenum returns also reduce the unit' s latent cooling capacity, which may be a critical aspect of system design in humid climates.


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34


Provide at least two 90° turns in return duct to avoid line of sight sound path back to space

RTU

Aim bypass damper discharge away _ ) .) from return inlet

1 Duct sensor -+- located upstream "' of bypass damper

Avoid return grilles in close proximity to bypass discharge air path //

,,/

Locate pickup in area of least turbulence, five duct diameters .,,,/ /'/ downstream of bypass ~ discharge air path

/~

/

~."'-....


Figure 42 Ceiling P lenum Return Layout

For ceiling plenum returns, be sure to keep return ceiling grilles out of the bypass discharge pathway. Cold supply air bypassed at partial cooling loads can fall through ceiling grilles causing discomfort complaints in zones affected. It also adversely affects zone temperature control so that some of the designed zone control is lost. Locate return I bypass junction a minimum of 15 ft from unit inlet

RTU

Avoid the temptation

~~- to install a short-circuit

bypass here. It will cause nuisance trips with zone capacity interruption.

Static Pressure ,A;_ Tubing


Figure 43 Ducted Return Layout


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When a ducted return is used, avoid a bypass location close to the unit. Figure 43 shows a proper layout for the manufacturing office if it had been designed with a ducted return rather than the ceiling plenum return. In terms of cost and ease of installation, the short circuit bypass location (right between the supply and return drops just below the rooftop unit) is tempting but the results are prohibitive. A void it. The return will mix poorly with bypassed air, causing nuisance machine shutdowns that leave the occupants without capacity until the machine controls reset. In addition, the environment inside the bypass ductwork so close to the unit, is very turbulent, making smooth, stable damper modulation difficult to achieve on direct-connected bypass designs. As with ceiling plenum return, locate the static pressure pickup downstream of the bypass takeoff:~ at least five (5) duct diameters if possible, and upstream of the first zone takeoff or main branch duct. Select an area of minimum supply air turbulence for accurate pressure sensing. Choosing a location upstream of the first duct branch ensures an accurate sampling of the entire system and not just ductwork feeding a portion of the building. A discharge air sensor is required to be mounted in the supply ductwork. Its function is to detect the changeover of the air source from heating to cooling or cooling to heating . This sensor needs to be mounted in the duct in a location where uniform discharge air temperatures will be sensed. The recommended location is in the horizontal supply duct a few duct diameters after the supply elbow, and it must be located before the bypass damper. If your system includes an economizer, be sure to include a backdraft damper in the return main ductwork just upstream of the tee joining the bypass and return ducts (Figure 43). During economizer operation, the machine's return damper modulates closed as its outdoor air damper opens. If the bypass opens when the machine's return damper is fully or partially closed, the bypassed air will take the path of least resistance. Without a return duct backdraft damper, the path of least resistance is to flow backwards through the return ductwork, dumping into the zones through the return grilles. Bypass feedback like this can be prevented by the backdraft damper. When using an economizer, also be sure to provide a source of relief to prevent over-pressurizing the building. Barometric or power exhaust may be used either on the air source unit or remotely located.

Damper Sizing Size the bypass damper using the following procedure: 1. Detennine the total cfin of your cooling and heating unit, based on block loads.

Installing the Byptiss Duct

and Dumper

2. Determine which is your smallest zone cfm. 3. Subtract the smallest zone cfm plus the minimum ventilation cfm of all other zones from your total unit cfm and this becomes your bypass cfm. 4. Never size for less than 75 percent of unit design airflow. 5. Size duct and damper the same.


Turn to the

36


In addition to subtracting the cfm for the smallest zone from the total unit cfm, some designers have also subtracted the sum of the minimum cfm required for each zone. This results in a smaller bypass damper and duct. It will probably work adequately as long as the minimum cfm configured at each zone remains unchanged or increases. But ifthe minimum cfm is reconfigured to a lower value sometime in the future , the bypass damper and duct will be undersized. Unacceptable noise and unstable machine operation may result. When in doubt, it is always safer and wiser to oversize the bypass damper and duct than to undersize it.

Diffuser Layout Another name for this step of design is room air distribution design. It involves choosing the type of diffusers and return grilles then sizing and locating them. Room air distribution effrctivc:ness is a rc:sult of good air mixing within the room, which is necessary to provide occupant comfort. Room air mixing diminates stratification, temperature variations, stagnant areas, and drafts. When this aspect of the VVT systc:m is negkctc:d, which it often is, the zone temperature control that VVT is designed to provide may fail to perform as designed. Then the customer will be unhappy because the zoned system they paid for provides very littk benefit.

I I

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2' " 4' Drop.kl Egg Crate Grille

-11-

I

I

.;.

I

-~

&-= 2' x 'Z DrosHo Egg Crate Grille

I~ ·

I I

I

0

I

I, I

I

I

I

"

I -at I

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- -

-

t

-

© li-~-

60'

., I

_Ill-_:... '®

&

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. .1:

100· - - - - - - - - - - - - - - -

--

I @

'-•

Figure 44 VVT System Room Air Distribution

The proper sdection of style, location, and size of supply air diffusers is the primary focus of good room air distribution design. It is also necessary to provide an adequate: number of return grilles and a clear pathway back to central returns. But proper diffuser design is what ensures that occupant comfort and adequate ventilation are provided. Errors in diffuser selection and location will never be corrected by the sizing and placement of return grilles. Think of the return grilles as a relief valve. The supply diffusers, on the other hand, pattern the room air.

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VARIABLE VOLUME AND TEMPERATURE ·--'"---·-----------------~

Diffusers are designed with specific performance parameters inherent to each style. The current ASHRAE Fundamentals Handbook provides recommendations for diffuser selection. Following the ADPI (Air Diffusion Performance Index) method will ensure uniform, comfortable air distribution. The designer of VVT systems cannot choose low-cost diffusers that dump supply air at partial cooling airflow or size them wrong or place them improperly and then expect to maintain zone comfort. Be sure to follow the diffuser manufacturer's guidelines for diffuser placement. However, diffuser and return grille location must coordinate with the ceiling layout and not interfere with the ceiling grid, lighting, or sprinkler heads and piping. A reflected ceiling plan is a great help in avoiding needless conflict and expensive surprises in air system design. Diffuser selection should be based on the throw, room shape, and room size. Select diffusers at both maximum and minimum occupied flow rates. For each zone, use the peak cfm that you obtained from the load calculations and decide on the number of diffusers needed in each zone based on the diffuser manufacturer's selection data. Here are five issues that must be considered in any design: • Diffuser cooling pe1formance • Diffuser heating performance • Occupant comfort • Acoustics (noise level) • IAQ (Indoor Air Quality) Nominal Length (ft)

Diffuser Type

3-Slot Director Diffuse r (OM)

1-Way Throw (ft)

1-Way Throw (ft) cfm

Min

5

4

2

Max

cfm

Min

Max

1-Way Throw (ft) cfm

Min

Max

50

2.0

9.0

100

2.0

13.0

150

3.0

18.0

75

3.0

14.0

150

5.0

19.0

200

5.0

24.0

100

5.0

19.0

200

8.0

26.0

250

8.0

30.0

125

7.0

22.0

250

11.0

31.0

300

11.0

34.0

150

9.0

24.0

300

13.0

34 .0

350

14.0

36.0

175

11.0

26.0

350

15.0

36.0

400

17.0

39.0

200

13.0

28 .0

400

18.0

39.0

450

19.0

41.0

-

-

-

-

-

500

20.0

44.0

-

Figure 45 VVF-Qualified Linear Slot Diffitser Rating, Heating Duty

Do not mistake dirt on the ceiling around the diffusers as a sign that filters need to be changed. Actually, dirt on the ceiling is a sign that the diffusers are working. The dirt is usually not coming from the air source but, rather, from room air induced by the high-velocity air jet entering at the diffuser. It deposits dirt from the room air at the low pressure point above the air jet near the diffuser. So smudging usually indicates a high induction diffuser and the need for improved routine cleaning of the carpets and floors.

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In most locations, the VVT system provides both heating and cooling from overhead, with a variable volume of air at the zone in both modes. This combination calls for a supply diffuser that is qualified for heating and cooling duty at full and reduced flow volumes. VA Vqualified linear slot diffusers work well for this application. When placed properly, they wash outside walls with warm air, forcing heating capacity down into the zone. They also mix air well in all modes of operation and blend in with tee-bar ceilings. An important quality is that they avoid dumping unmixed, cold, supply air directly into the conditioned space when the airflow is reduced at partial cooling loads.

3-Slot Director-Diffuser

Standard 2-Way Diffuser

Linear slot boot diffuser

Figure 46 Use linear slot VVT-qualified diffirsers for best p e1formance.

For the manufacturing office (Figure 2), which is located in Newark, New Jersey, a 3-slot, director diffuser was selected for perimeter areas and a standard, two-way linear slot diffuser for the core. The director-diffuser is a special linear slot diffuser also knmvn as a ''flip-flop" or "changeover" diffuser. It has a heat-sensitive diverter that directs all the air in one direction when heated air is being delivered to the zone. That way, the heated air can be forced down the outside wall in heating but will return to two-way discharge when the system switches back to the cooling mode. Sound level specifications for selecting diffusers should be based on clearly stated assumptions and should reflect real project needs, not any manufacturer' s data. Use cum:ntly accepted space application factors. Bringing the required space sound level down unrealistically low increases initial costs and may hinder proper diffuser performance by forcing low diffuser velocities at peak airtlow, thereby compromising performance at off-peak conditions. Oversized diffusers have a low sound level but dump cold air more readily than properly sized diffusers. Return grille placement has little impact on the pattern of air motion throughout the conditioned space; because, unlike a supply diffuser, the velocity profile at the inlet of a return grille rapidly drops off within only a few inches of its surface. So, be sure to provide adequate opening area for return grilles. A 2 ft x 2 ft egg crate ceiling grille can handle around 2000 cfm (5 tons cooling capacity) when running at 533 fpm (feet per minute) over the net grille area.

Adequately sized ""'/ flex duct_ ".. 1,--,,.11,....,.;.,..,,.,.,.,---,1 I

Return Grille _ /

l

Area that communicates with central return

f

r

Sheet metal return box with acoustical duct liner

CE ILING

" -Return Grille

Isolated Space

Figure 47 Overhead Return Air Transfer Loop

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A central return grille will work well if a clear pathway is provided for air to find its way back to the return grille from other areas. However, undercut doors provide little passageway for return air. A Yz-inch undercut can handle only around 70 cfm of return air when closed. This is inadequate for most offices, even if they are small. In addition, undercutting the door compromises acoustical privacy when the door is shut. An alternative for getting air from spaces with a closable door back to the central return is to provide overhead transfer ductwork. It has a grille at the ceiling of the room and one in the open area outside the room. Flex duct connects the two so that supply air from the room moves through the transfer duct into the adjacent area, then back to the central return grille. It will work well even when the doors are shut, while at the same time, providing acoustical privacy. Be sure to size the flexible duct adequately. It will be larger than an equivalent sheet metal duct. There are new construction cases when this approach will be more cost-effective than individual returns installed in each room. In the aftermarket, it is often the best method when the partitioning changes.

Control System Details VVT systems contain unique control algorithms that were specifically developed to properly switch the HVAC unit over from cooling to heating mode, and vice versa, while maintaining required set points throughout the system. These Air source unit controller algorithms are housed in a zone controller located at a zone damper : 24vac or m a bypass 4Qva System Pilot "!:: controller located at the bypass damper. I

I

I

:

I

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ZC/C02 {Optional for DCV)

Figure 48 Typical VVT System

Linkage Coordinator versus Standard Zone Controllers A complete VVT system will have numerous zone controllers arranged in a linkage coordinator/zone controller relationship. Any zone controller can be configured as the linkage coordinator and is capable of coordinating up to 31 additional zone controllers. Setting up the linkage coordinator and the zone controllers it controls is a matter of proper configuration and addressing at the time of system installation and start-up.

<(f,. Commercial HVAC Systems

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The linkage coordinator determines the requirements of the complete VVT zoning system and passes those requirements on to the air source controller. The air source will respond with its current air source mode. The linkage coordinator zone controller will rday this information to each zone controller on its address list. For stand-alone operation, the linkage coordinator can determine an air source mode without actually communicating with it. Each zone controller is responsible for the temperature control of its zone . Zone controllers determine the proper quantity of airflow required to bring the space back to set point. The airflow is modulated based on a percent of wide-open damper position. Each zone controller includes an integrated, floating point actuator. A floating point actuator can come to rest at any point of the actuator's range, from fully open to fully closed. It uses actuator power to open and close the damper. It does not actuate in one direction and rely on spring return in the opposite direction, as some actuators do . Thus, if there is a power failure, the dampers will be in the position they were in when the failure occurred.

Bypass Controller The bypass controller works in conjunction with a bypass damper to act as the pressure regulator of the system (Figure 4). This function is necessary because zoning systems present variable loads to constant volume equipment in a VVT syst<.:m. If a majority of the zone dampers close simultaneously and there is no bypass system, the supply air system static pressure will rise as the airflow decreases. This condition can damage both the HVAC equipment and system ductwork. The function of the bypass controller is to measure the pressure in the supply airstream and modulate the bypass damper to maintain a configured set point. The bypass controller must be field-mounted on the bypass damper. Multiple actuator sizes are available to meet static pressure and airflow needs of any job.

The System Pilot All VVT systems require at least one system pilot user interface to provide access to the VVT system as well as any other device residing on the communication bus. The system pilot is used to configure, commission, balance and start-up any controller on the bus. The system pilot can act as a real time broadcaster for the system. The system pilot is also useful for monitoring the system at any time.

Th.i W_l llt5aAM

The system pilot has a temperature sensing thermistor that can be used to control zone temperature in the zone where it is located. The system pilot may also be mounted in a location remote from the zone controller it is associated with and a zone sensor in the controlled zone used to sense temperature. Additional system pilots may be attached in other zones. With proper configuration and addressing, they can provide Figure 49 space temperature set point adJ·ustment, initiate occupancy system p·zzot override, and view the mode for each zone when: they are installed. In this application, the system pilot can be wired directly to the VVT zone controller at its local communication port.

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Commercial HVA C Systems

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-

ROOF

I

I I I I

II

-

'--. ----, I

1

~------~

Pl~~

ZO

LC

•''

-

RG

•'

~

SW ~ /so ...........

RG

Ci)

SP

ZS

ZO

ZO

ZC

.....~----.-=~~w....._~.......,-~--+~

w.._~..._.....,-~=.---w~~

~: sw /so ...........

-

----------, . ----------, SW~ / s o '

RG

RG

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Ci)

ZS

..

ZS

SP/ZS

@ • Set point • Set point adjustment • Occupancy override •View mode

Figure 50 Multiple System Pilots

Space Sensor Locations and Options The location of room sensors (Figures 2 and 4) is critical to the reliable operation of a zoned system like VVT. Room sensors should be located about 5 feet above the floor on an interior wall that does not receive direct solar radiation. Avoid locations where supply air discharges directly on the sensor and stay away from heat generating devices such as computers, fax machines, copy machines, coffee pots, microwave ovens, etc. Space temperature sensors are required in each zone. The system pilot may also be used to sense space temperature. These are thermistor-based temperature sensing devices whose resistance value changes as the zone temperature changes. These devices are available in a number of different configurations to meet the requirements of the zone. They can be provided as temperature sensing, temperature sensing with an override switch, temperature sensing with an override switch and set point adjustment, and also with all these functions and a display of the zone ' s current temperature. The override switch allows initiating a temporary occupied period when the system is in unoccupied mode. The set point adjustment allows modifying the zone's current set point temperature in the zone.

Figure 51 Space Temperature Sensors

Thennistor-based space temperature sensors are cost-effective devices. The simplest version has no set point adjustment. Optional slide-bar set point adjustment and occupancy override is also available. This allows the occupants to adjust the temperature set point and override an unoccupied mode of operation temporarily. Room sensors are also available with liquid crystal display (LCD) of space temperature.

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Combined Space Temperature and C02 Sensing In addition to space temperature, combined temperature/C0 2 sensors that monitor carbon dioxide levels in the zone are also available. The level of C0 2 in a space is a good indication of the number of people and can be used to adjust the ventilation air to assure proper quantities are introduced into the space. C0 2 sensors provide zone level demand controlled ventilation (DCV) and meet the requirements of ASHRAE Standard 62 (Ventilation for Acceptable Indoor Air Quality). More will follow on DCV later in this module . Figure 52

-

Space Temperature/C0 2 Sensors

Humidity Sensor

A humidity sensor may be used in place of the C0 2 sensor to monitor the zone ' s humidity level. Since no cooling coil is located in the space at the space level, the control only serves as a monitoring device. However, when the sensor is used with an air source unit with humidity control options, the sensor can be used as input to control the humidity control device. Additional information on the use of these devices can be found in TDP-631 , Rooftops, Level 1, Constant Volume.

Zone Sensor Averaging Zone sensor averaging is possible for larger control areas where an accurate temperature reading is not possible using a single sensor. An example application is an office area with cubicles separated by half partitions. The temperature in one cubicle may be somewhat different than another and neighboring occupants' ideas about a comfortable temperature may not be the same. Sensor averaging in this situation will help provide a reasonable degree of comfort across a larger space. It is possible to averagi:: the system pilot with 4, or 9 sensors only in a series or parallel configuration. For added flexibility, a system pilot may also be averaged with 1, 4, or 9 sensors providing sensor averagmg.

Sensors Only

~""

Sensors With Optional System Pilot 1 Thermistor Sensor

~ qjJJ .

-

- ~L___f [JJ ·~

4 Thermistor Sensors

uJL[::jJJ

9 Thermistor Sensors ·

_ -.J-~

Figure 53 Zone Sensor .Averaging

Outside Air Temperature Sensor Outside air temperature sensors are required for proper VVT system operation. Some sensors are included with the equipment. Others must be ordered as a hardware accessory for the job. Depending on the air source that is used, an outside air temperature sensor may be factoryinstalled. Refer to the packaged rooftop, indoor package, or split system literature for verification .

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VARIABLE VOLUME AND TEMPERATURE On typical VVT jobs that use a rooftop unit with a factory-installed air source controller, the outside air temperature sensor will be provided from the factory. For field-installed applications using a packaged rooftop unit, a good location for the outside air temperature sensor is underneath the outdoor air intake hood. The sensor should be connected to the air source controller by shielded, 2-conductor, low voltage cable, just like that used for zone tstat/sensors. As an alternative, 2 of the 3 wires of bus conductor can be used to standardize the wiring used on the job.

Zone Level Demand Controlled Ventilation (DCV) DCV is a real-time, occupancyI I 11 II I I I I I I I I I 100 ·--.._: I I I 11, / / / / / , r / / '-......... '- "-.'\.' "'\ ' based ventilation control approach c 90 // ..__ .g 80 that can offer significant energy savi .....- >---~ 70 ~ 60 ings and improved ventilation over :;;; 50 traditional fixed ventilation ap' -~ 40 .....- v ~ 30 ,, proaches. Properly applied, it provides ..__ >---b 20 v ......_ >---, ..... / / " the ASHRAE prescriptive ventilation ~ 10 .................... / / / 11 / ' \ \"' '\ 0 rates at all times. Even in spaces I II II I I I II I 0 2 4 6 8 10 12 14 16 18 20 22 24 where occupancy is static, DCV can Hour of Day be used to ensure that ewry zone • Constant Ventilation within a space is adequately ventilated • Zoned C02 Control Ventilation for its actual occupancy. Air intake D Energy Saving dampers, often subject to maladjustment or arbitrary adjustments over time, can be controlled automatically, Figure 54 Demand Controlled Ventilation: JAQ and Energy Efficiency avoiding over or under ventilation.

-

f-f--

f--

'

......_~

----

/----

>----

"/

-

~~

-

-

Zoning Systems with DCV Typically, the minimum cooling and heating percent damper position set points are configured to provide the proper ventilation for a space with an average level of occupancy or to meet the building ventilation requirement. As people enter the space, the demand for ventilation will increase as more occupants expel C0 2 . The zone controller will respond to the increased level of C0 2 by increasing the airflow into the space. So C0 2 sensing is an indirect way of following the occupants around the building and providing them with adequate outdoor air ventilation. To provide DCV, the zone controller will monitor the ventilation of the space through a C0 2 sensor. The sensor will be used to determine if adequate ventilation is being provided by measuring the C0 2 level at the tt:mperature/C0 2 sensor in parts per million (PPM). If the level rises above a configured maximum set point (e.g. 1000 ppm), that means ventilation is inadequate. The zone controller will calculate a new minimum percent damper open position set point to maintain the C0 2 level at the desired set point. This new minimum percent damper open position set point required for adequate ventilation will override the damper position set point required for space temperature control, resulting in an increased supply airflow and ultimately a decrease in C0 2 level because the higher airflow to the zone carries more outdoor air with it. While providing conditioned air at an increased minimum damper position can cause over cooling in some instances, the system can change over from heating to cooling and vice versa rather rapidly as it works to provide adequate ventilation and precise temperature control.

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VARIABLE VOLUME AND TEMPERATURE The linkage coordinator may collect the individual C0 2 concentration readings from its associated zone controllers and relay the info1mation to the rooftop to have the rooftop increase the ventilation rate at the air economizer if this is necessary to satisfy the ventilation requirement. The rooftop can also temper the outdoor air at extreme conditions so the ventilation air introduced into the system does not cause a cold draft situation to occur. For more information on designing with demand control ventilation, see TDP-901 , Indoor Air Quality, or Demand Control Ventilation Handbook.

Wiring and Power Requirements The VVT control system receives its power to operate through the zone damper power circuits. It is not a battery-powered system nor is there battery backup. The entire system is powered by 24 volts AC. If a power failure occurs, all schedules and programming in all zones stay indefinitely in a nonvolatile memory chip. Each damper receives its power from a 24-volt transformer supplied at or near the damper. The primary side of these transformers normally uses 120-volt power from a dedicated house circuit, usually in the ceiling, that must be provided by the electrical contractor. A junction box should be provided above each damper location for mounting the 24-volt transfo1mer.

System Options Since VVT systems use DDC controls, a number of other system operating features are possible and are being provided by manufacturers of these systems. These features fall into four general categories: energy conservation, support of fire and life safety, diagnostics and alarm monitoring, and systems networking. For example, to save energy when outdoor conditions are suitable during unoccupied periods, the system can enter economizer operating mode with the dampers modulating between cooling and heating set points, which will cool the building to occupied cooling levels - this is referred to as night time free cooling . To limit the electrical demand, the system can stagger the start of units and accept a demand-limiting signal. As is required by ASHRAE 90. l , the system can optimize start time to bring the air source unit from unoccupied to occupied only as early as is required to meet the actual heating or cooling demand, called optimized start. The system can also be configured to purge the building of hot stale air before an occupancy period begins. Finally, the system can use error reduction strategies to minimize over-compensation of the heating or cooling system. Some features are incorporated into systems that assist in providing fire and life safety control. When commanded by a fire control station, the zoning system can function with the air source unit to go into a pressurization mode in which all zone dampers drive to the full open position allowing excess air into the zone to help drive smoke from an occupied zone. The dampers can also be driven fully closed with the exhaust fans pulling air from the zone providing a smoke evacuation mode. The DDC control features also allow the system to perfom1 diagnostic checks on itself and to provide monitoring of alarm or error conditions. Since it is a DDC network, the system can connect to a building system network, allowing control and monitoring of the system and many other systems in a building or complex from one location on or remote to the site.


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Supplemental and Perimeter Heat The VVT system is designed to provide a central source of heating and cooling capacity. However, it cannot provide a central source of heating and cooling at the same time. In view of this limitation, the way to approach a job with a simultaneous need for heating and cooling at design winter conditions is to provide Heat dedicated heating in critical heating or zones. '-,,_,_

Cool

.....

;::i.....

.....

~

Heat then Cool

.....

For instance, the manufacturing office (Figure 2) has a cooling load in the core zone (zone 7) most of the year whenever the space is occupied and lighted with computers on. Technically, it is not a critical zone for Figure 55 cooling because it looses its cooling VVT central units can provide heating or cooling, but not both al the load below around 26° F outdoors. same time. Zones with different load patterns will exhibit different changeover temperatures. Perform a load calculation on unique zones to determine the difference from the other zones. However, zone 7 has some of the qualities of a critical zone because of its relatively constant cooling load even when other zones may require heating. This means that during the winter some or all of the perimeter zones may require some degree of heating while the core zone requires off-peak cooling. Since cooling cannot be manufactured locally, duct-mounted zone hot water heating coils were selected for all perimeter zone VVT damper units. One each will be installed at the discharge of the dampers for zones 1 through 6. This way, cooling can remain on for the core while the perimeter zones calling for heating modulate to their minimum flow setting and turn on the heating coil. As an example of a building with a critical heating zone, consider an office with a front entrance that has a lot of glass exposure and a vaulted ceiling. Installing perimeter baseboard heat for this zone will satisfy the zone heating needs and avoid this zone perpetually calling for heat from the VVT system when all or most of the other zones are calling for cooling. This way, the critical zone can be included with the other zones for VVT control, but the unusual heating need will be satisfied by the baseboard heat.

Hot water or electric reheat coil

Factory-mounted Damper Controller Actuator (minimum desired position must be configured)

Figure 56 Typical Supplementary Heat for Manufacturing Office

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When a zone requires cooling year-round, like a core area with no roof (a computer room, video studio, etc.), it is best to treat it with a separate system. Such a zone is a critical cooling zone. It is critical because it has special needs that other zones in the building do not have. It is also critical because its special needs cannot be ignored and they become a driving influence in system design decisions. Systems handle a critical zone best when they have a separate HVAC unit dedicated to each zone. Such systems include PTAC units, multiple packaged units, and water source heat pumps . All-air systems, like VVT and VA V, that use a single central HVAC for multiple zones have a more difficult time dealing with a critical zone. When all zones require some degree of cooling, the central HVAC unit remains in the cooling or economizer mode . When all zones require heating, the central unit stays in the heating mode . When outside temperatures are intermediate with some zones requiring heating and others requiring cooling, the VVT system switches the central unit back and forth from heating to cooling to alternately satisfy heating and cooling zone loads that coexist. However, as the use of VVT systems has grown, the demand for simultaneous heating and cooling capacity has risen. When a zone is waiting for the capacity it needs but the central unit is in the opposite mode, the zone damper is modulated to the minimum damper position, maintaining its source of ventilation and room air motion. As a result, the zone temperature may drift unacceptably. If the zone requires heating, supplementary heat usually solves the problem. If the designer wishes, heat can be provided by using a fan-powered mixing box. An optional control board installed in the zone controller enables the control of auxiliary fan and heating coil. The auxiliary heat may be either ducted or nonducted.

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When it is ducted, the heat source is located within the air terminal (e.g. fan-powered mixing box), upstream of the supply air temperature sensor. A hot water heating coil may be used or up to three stages of electric heat. Applying heat this way requires the terminal to contain a supply air temperature sensor. The sensor will provide feedback to the auxiliary heating control loop and will allow the zone controller to ensure that the supply air heating temperature stays within the configured maximum setting.

Air source unit controller

Duct sensor (locate upstream ofdampe, System Pilot

I

1

24vac : 40va

20/2/s cable"[

::t; I

:

I

I ZS/C02 · (Optional for DCV)

Figure 57 VVT Pressure -Independent (PD) or Pressure-Independent (PI) with Fan-Powered Zones and/or Reheat System.

When a fan-powered mixing box is not used, ducted auxiliary heat can only be used when the air source is on, since the air source is providing the only means of airflow into the zone. When used in either a series or parallel fan-powered mixing box, ducted auxiliary heat can be used when the air source is off or on. When the air source is off, auxiliary heat will supply unoccupied heating capacity by cycling the terminal fan to provide airflow to the zone. When supplemental heat is non-ducted, the heating capacity is located in the conditioned space. Hot water, electric and steam baseboard heaters, or radiant panels may be used. In these applications, the zone controller typically provides only on/off control of a two-position heating valve or a single-stage electric heater. No heating control loop feedback sensor will be required. The zone controller will only use space temperature as feedback. However, the zone controller can support a hot water baseboard heater with a modulated hot water valve if the water sensor is used for feedback on the leaving water temperature of the heater. In this case, the baseboard heater will be controlled in the same manner as a modulating ducted heater.

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In other cases, the baseboard heat may be the first stage and ducted heat is installed for the second, or second and third stages. Space temperature will be used as the feedback for the baseboard or radiant heat (first stage) and the supply air temperature, along with space temperature, can be used for control of the second and third stages. This zone upgrade allows for great flexibility in applying VVT zoning in spaces that can have substantial heating loads, such as lobbies with lots of glass.

Summary The objective of this module has been to learn the concept and benefits of VVT systems and the complete operating logic and sequence of control. VVT is a variable volume and temperature multizone system that uses a central constant volume HVAC unit to satisfy up to 32 individual comfort zones. The VVT components include the bypass system, zone dampers, linkage coordinator, zone and bypass controllers, space sensors, and necessary safeties to protect the system. The zone damper tstat/sensor in combination provides a higher degree of comfort to building occupants from a cost effective, single zone heating/cooling unit that functions as a multiple-zone unit. VVT with DCV provides the building owner with both comfort and a code-compliant, indoor air quality solution. The new generation system utilizes C0 2 sensors in the space to do zone level demand controlled ventilation. Improved air filtration is available since the filters on a central unit are usually more efficient than those on a chilled water room fan coil, duct-free split system fan coil, or water source heat pump. The VVT system uses lower pressure class ductwork, but it is imperative that the VVT ductwork not be undersized in order to avoid noise and static pressure buildup problems. Equal friction, static regain, or modified equal friction methods (as detailed in TDP-504, Duct Design, Level 1, Fundamentals) can be used for sizing the ductwork. Rectangular or round zone dampers can be used. A bypass system is required for each system to maintain the static pressure in the duct and to maintain minimum airflow through the central I-IV AC equipment. Sizing the bypass requires only sizing the damper for 75 percent of the design airflow. The user-defined linkage coordinator will continuously monitor all th~ zones of the VVT system and choose the system mode of operation based on the requirements of the reference zone. The linkage coordinator will communicate with the HVAC unit via linkage through the microprocessor controller used to control the HVAC unit. Each zone will require either a space temperature sensor or temperature/C0 2 combination sensor for DCV control, or the system pilot. Every system will require a minimum of one system pilot. The system pilot allows the user to configure, commission, balance, start-up, and view system and zone data from a centralized point. It is essential for the designer to have a clear understanding of all the VVT system components to make correct VVT zoning design decisions.


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Work Session 1.

What is linkage? _ _ _ _ _ _ _ _ _ _ __ _ _ __ _ __ _ _ _ _ _ __

2.

True or False? VVT systems must have a direct connected bypass duct from supply to return. - - - - - - - - -

3. What is the difference between a pressure-dependent and a pressure-independent zone control strategy?

4. What is the reference zone - - - - - - - -- - --

-

----------

5. Why is a bypass damper necessary for the VVT system?

6.

Why is a variable volume unit (including VFD) not required in VVT systems?

7.

True or False? VVT Systems can be applied on large buildings of several hundred ton size using a single HVAC unit. _ _ _ _ __ _ __

8.

True or False? A building can be designed with zones without the use of a zoned system.

9. List two guidelines for sizing bypass:

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10. List five typical factors that create the need for a temperature control zone.

11. Define a control zone:

12. List the three types of central HVAC equipment usually used on VVT systems.

13. List five systems that compete with VVT:

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Engineering Design Steps

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D Figure block cooling/heating loads D Do preliminary equipment selection D D D D D D D 0 D D D D D D D


Designer Checklist

Determine control zones Determine zone cooling I heating loads and cfm Select and position supply diffusers Select and position return grilles Select and position VVT dampers Layout duct system Layout bypass system Position temperature sensors and system pilot(s) Size ductwork Select supplementary heat if needed

Make final equipment selection (determine if add-on air source controller is required) Select accessories (e.g. Outside air temperature, C0 2 or RH, PAT, DAT sensors) Define control and power wiring requirements and routing Review VVT system installation notes to contractors Have the contractor complete the VVT Installation Start-up Request Checklist

Electrical Items Make sure the electrical design provides for the following:

D

A disconnect within sight of the HVAC unit or air source unit (provided on the unit, if available, or supplied by the electrical contractor) with a fuse or HACR type breaker protection provided in the power line serving the unit.

D

A junction box and a 24VAC, 40VA control voltage transformer for each zone damper controller.

D D D D

A separate 24VAC, 40VA control voltage transformer for the bypass system controller. Specify that all VVT control wiring be shielded type only. Specify that all VVT control wiring in a ceiling plenum return be plenum-rated shielded wire. _ Specify that any VVT control wiring in conduit not be run in the same conduit as other AC wiring.

Ductwork and Dampers

D

Specify clearly all zone damper and bypass damper locations, shapes (rectangular or round) and sizes.

D

Allow room after the bypass takeoff but before the first zone takeoff for the bypass pressure pickup to be located.

D D D

Specify balancing dampers where more than one supply diffuser is fed by a zone damper.

D

Show on plan the access doors to each damper if ceilings are inaccessible type. Specify that return grilles in ceiling not be located near bypass duct discharge on ceiling plenum return jobs. Coordinate architect's ceiling type with diffuser vendor prior to diffuser layout and bidding.

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Sensors

0

Clearly show all zone sensors in correct locations.

0 Determine the best location for the system pilot(s) based upon its intended usage. Cl Determine which zone will function as the linkage coordinator and provide proper sensor.

0

Clearly indicate duct mounting location for the duct temperature sensor.

0

Determine if outdoor air and primary air sensors are required and clearly show locations on the drawings.

Miscellaneous

0

Specify at bid time who provides the personal computer if computer monitoring via CCN sofhvare is desired.

0

Specify start-up and programming assistance to be supplied by personnel from the VVT manufacturer. This is to be done after all wiring connections are made and the system is ready to program.

VVT System Don'ts 1.

Don 't zone constant load areas on variable load systems or those that require VA V control schemes, such as: • Computer room zone on a RTU serving offices • Morning warm-up with cooling-only VAV boxes • Supply air temperature reset

2.

Don 't undersize the zone dampers. This will cause potential noise in your system.

3.

Don ' t undersize the bypass damper and duct. This will cause potential noise in your system and sluggish system response to zone load changes.

4.

Don't design a direct, ducted bypass on ducted return systems without room for mixing return and bypassed air. The bypass should meet the return duct at least 15 feet from the unit return. Use a backdraft damper in the return main upstream of the junction of the return and bypass ducts to avoid bypass backing up in the return duct and dumping out the return grilles.

5. Don't design VVT zoning on equipment over approximately 25 tons. Use a VAV system or multiple VVT systems. 6.

Don' t design a system with too many small zones. For example, a 15-ton system with all 6-inch zoning dampers becomes a noisy system.

53


«

Installation Notes for Contractors

General Wiring Guidelines: l.

Always use shielded 20AWG stranded wire for all control wiring.

2.

Always use plenum-rated cable when running wire outside of raceways in ceiling plenum return.

3.

Always isolate wiring from cables carrying AC voltage.

4.

Always install lightning suppressers when communication bus leaves the building.

Communication Bus Wiring and Layout: l.

Always use shielded 3-conductor 20AWG stranded wire for all communication wiring.

2.

Always use plenum-rated cable when running wire outside of conduit in ceiling plenum.

3.

Always run communication bus wiring in parallel daisy chain between linkage coordinator, zone, and bypass controllers.

4.

Always keep communication bus free of T-taps, junctions or spurs.

5.

Do not bundle communication bus cable with other control or voltage-carrying conductors.

6.

Do not connect more than 128 VVT devices to one communication bus and no more than 32 zone controllers per system (you can have more than one system per bus). Use a CCN Bridge to extend the bus if needed.

7.

Do not connect more than 8 VVT scanning devices to one communication bus; use a CCN Bridge to extend the bus.

8.

Do not allow communication bus length to exceed 4000 feet; use a CCN Bridge or repeaters to extend the bus.

9.

If communication bus length is over 1000 feet, a repeater may be required.

10. If the communication bus requires tie-in to existing or new CCN bus, consider adding a CCN Bridge.

Power: 1.

Provide a 24VAC/40VA dedicated power transformer to each zone controller

2.

Avoid using common transfonners to power VVT zone controllers.

Actuators: 1.

Make sure damper airflow arrows are correct when installing dampers. Mark damper shafts at installation with a line to show damper position in duct.

2.

Size zone and bypass dampers for the engineered airflow of the HVAC equipment.

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54


Bypass System: 1.

Use rectangular bypass dampers whenever possible to simplify installation. Match damper and duct size. Where round is used, minimize round-to-rectangular transitions.

2.

Mount the static pressure pickup securely at least five (5) duct diameters downstream of the Bypass Damper in the main supply duct, but upstream of the first branch or zone rnnout duct.

3.

For ducted return systems, do not design the bypass ductwork to be a short duct between the supply and return mains. Adequate mixing will not occur.

4.

Size bypass ductwork to maintain proper equipment operation when all zones are closed except the smallest. It is common for customers to require the smallest zone to operate when all others are satisfied. Design bypass cfm = HVAC unit design cfm minus smallest zone cfm and the minimum ventilation of all other zones, but> 75% unit cfm.

5.

Use single bypass damper wherever possible. Select torque of actuator to match damper.

6.

Where supply plenum is used, manifold multiple static pressure pickups in the supply duct branches leaving the plenum, and locate pickups at least 5 duct diameters downstream from the plenum.

Generally Wise to Do: 1.

Hire an air balancer for start-up.

2.

Consider dedicated HVAC cooling systems for interior spaces with unusually high cooling loads, such as computer or copier rooms. Likewise, areas that have an unusually high heat loss, like vestibules, should incorporate alternative heating sources.

3.

Try not to use VVT to balance the system as a means of design . Rather, use VVT to fine tune the system if design is inadequate.

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Quantity of Systems: VVT Linkage Coordinator Model Numbers: - - - - -- -- - - -- V VT Bypass Controller Model Number(s): _ __ _ __ __ _ __ _ VVT Zone Controller Model Numbers: VVT Air Source Controller Model Number(s): - - - -- - - - -- -Stat ic Pressure Sensor Model Number(s): Quantity of Linkage Coordinators : (1 per HVAC unit) _ _ _ _ _ __ _ Duct Temperature Sensor (1 required) _ __ _ _ _ __ _ _ _ __ __ Number of Zones: - -- - -- - - - -- - -- - -- - - - - - Number of Bypasses: Number of C0 2 Sensors: Please list all additional sensors used:

VVT Installation Start-up Request Checklist

-------------~

General Wiring Yes / No Yes / No Yes / No Yes / No Yes I No Yes / No Yes / No Yes / No Yes / No Yes / No

1.

2. 3. 4. 5.

6. 7.

8. 9.

10.

Shielded 20A WG wire used for all control wiring. All shields tied together to fom1 a continuous electrical shield connection. Wire shield grounded at one single location and taped off at all others to prevent grounding. Lightning suppressers installed when bus leaves building. All wiring isolated from cables carrying AC voltage. Wiring verified to be free of short circuits. All wiring connections tight and per the wiring diagrams contained within the installation manuals of the equipment manufactun.:r. All controllers wired and securely mounted. Air source controller wired to the HVAC equipment (if not factory-provided). All processor board cables and tem1inals tight.

Communication Bus Wiring and Layout: 1.

Yes / No Yes / No

2.

Yes / No Yes / No

3. 4.

Yes / No

5.

Yes / No

6.

Yes / No Yes / No

7.

8.

All wiring shielded and meets general wiring requirements shown above . Communication bus wiring is parallel daisy chained per the instruction manuals from the equipment manufacturer. Communication bus free of T-taps, junctions and spurs. Communication bus cable not bundled with other control or voltage-carrying conductors. There are no more then 128 VVT devices connected to one communication bus, with maximum 32 zone controllers and 8 scanning devices per system. There are no more than 8 VVT linkage coordinators connected to one communication bus. Communication bus length does not exceed 4000 feet. Communication bus length is over 1000 feet (requires repeaters at every 1000 ft segment).

.,



56


Power: Yes / No Yes / No Yes / No Yes / No

1.

24VAC/40VA minimum power to the air source controller.

2.

24VAC/40VAminimum dedicated power transformer to each zone controller. All 24 VAC power supplies verified complete and secondary voltages tested . HVAC equipment power supply complete and tested.

3.

4.

Actuators: Yes I No Yes / No Yes / No Yes / No Yes / No

1. All dampers free of obstruction. 2. All actuators connected to damper assemblies and securely mounted. 3. Damper airflow arrows are correct and damper shafts are marked to show damper position. 4. Zone and bypass dampers sized for engineered airflow of the I IVAC equipment. 5. Zone actuators wired per the manufacturer's Installation Manual.

Bypass System Information: Yes I No

1.

Supply duct static pressure required by the equipment for engineered design: _ _ _ _ _ _ _ _ _ _1n.wg

Yes I No

2.

Yes I No

3.

Yes I No

4.

Yes I No

5.

Yes I No

6.

Static pressure pickup securely mounted at least five (5) duct diameters downstream of the bypass damper in the main supply duct trunk and before the first branch or zone runout duct. Connect to high side pmt with tubing. Bypass ductwork is sized to maintain proper equipment operation. HVAC unit design cfin minus smallest zone cfin, but not less than 75 percent of HVAC unit design cfin. Multiple interconnected bypass unit controllers wired per the manufacturer's instruction manual. Static pressure sensor low port connected via tubing to a non-conditioned space, which is not affected by the pressure of the system. Duct temperature sensor mounted in the supply duct before the bypass and wired to the bypass controller.

HVAC Equipment: Yes I No Yes / No Yes / No Yes / No

Type of HV AC equipment: _ _ _ _ _ _ _ _ _ _ _ __ 2. Model number of HVAC equipment: _ _ _ _ _ _ _ __ 3. Equipment test run and ready for start-up: _ __ _ _ __ 4. Supplemental heat controlled by this system (if yes, please explain): 1.

Other: Yes / No Yes / No Yes / No Yes / No

System air balancer notified of start date and time. 2. Customer notified of start date and time. 3. Installing VVT and HVAC contractor technician notified to be on job site for start-up (required, not optional). 4. Job engineering blueprints and specifications with sequence of operations sent to commissioning agent. 1.

Commercial HVAC _ _____ _ _Systems _ _ _ _ __

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® _ _ __ _ _ _ _ _ _ _ _ _ _ _ _ _ Turn to the Experts.

57


Work Session Answers 1.

Linkage refers to the process through which data is exchanged between the air tenninals and the air source device that provides the supply air to those tenninals. The process "links" the tenninals with the air source to form a coordinated system.

2.

False. The bypass duct can discharge air into a ceiling plenum return. In fact, this is a preferred method since mixing of the bypassed air and plenum air prevents the divert introduction of conditioned bypass air into the unit return.

3.

Pressure-dependent zones modulate their zone damper position in percentages to maintain a temperature set point (e.g. 65 percent open). A pressure-independent zone modulate the damper position based on actual desired airllow (e.g. 350 cfm). The position of the damper is irrelevant. The zone damper assembly will include a pressure sensor and an airllow measuring device when pressure-independent control is used.

4.

The reference zone is the comfort zone requiring the greatest heating or cooling demand for the longest period of time.

5.

A bypass damper is required to maintain a reasonably low static pressure in the supply duct and to maintain ininimum safe airllow through the constant volume HVAC equipment used with VVT.

6.

Because a VVT system incorporates a bypass to maintain proper airllow for a constant volume unit.

7.

False. It is best to zone a large building first with multiple packaged units no larger than approximately 25 tons each, then subzone within each unit with VVT systems.

8.

True, by using multiple constant volume, single zone units, PTAC, duct-free splits, or water source heat pumps.

9.

Never size less than 75% of unit design cfm Size bypass to equal unit airllow minus smallest zone airllow and the sum of ventilation air of all other zones.

10. Any five of the answers below : Space usage

Lighting control zone

Wall exposure (area and orientation)

Glass exposure (ft2 and % wall area)

Lighting level

Tenant variations (schedule, preference, billing)

Glass orientation (N, E, S, W)

Perimeter versus core exposure

Special ventilation needs

Occupant schedule

Roof exposure or none

Special exhaust needs

People density

Occupant responsibility (authority in building management)

Special air cleanliness (IAQ) requirements

11. A control zone is a whole building, group of rooms, single room, or part of a room controlled by its own thermostat or temperature sensor. 12. Packaged rooftop unit, Split system, Vertical packaged unit (air or water-cooled), VVT versions of water source heat pump 13. Multiple rooftops (indoor package units, or split systems), PTA Cs, WSHPs without VVT controls, Duct-free split systems, Room fan coils (chilled water) and VA V

•+@•1


®

Tmn to the Experts.

58

Prerequisites: To obtain the highest benefit from this module , it is suggested that participants have prerequisite knowledge from the TDPs listed below, or equivalent. Book

TDP No.

TDP-103 TDP-504 TDP-701 TDP-801

Cat. No. 796-027 796-045 06-796-067 796-074

Instructor CD Cat. No.

797-027 797-045 06-797-067 797-074

Title

Concepts of Air Conditioning Duct Design, Level 1, Fundamentals System Selection Controls, Level 1, Fundamentals

Learning Objectives: After reading this module, participants will be able to : • Make a simple schematic sketch of a VVT System and label the components • Define a temperature control zone . • List the three types of central HVAC equipment that are usually used on VVT systems and describe each type . • Explain how a building can be zoned without using a zoned system. • Give two reasons why a zoned system makes more sense than a modular layout using single zone, constant volume units for the manufacturing office . • Explain the meaning of VVT • Sketch and describe how the VVT bypass damper and duct results in variable temperature at reduced load . • List five systems that compete with VVT. • Use pie charts to refine exposure and occupancy zoning decisions . • List the components required to complete a typical VVT system . • Describe how the central HVAC unit accomplishes changeover from heat to cool (and vice versa) in a VVT system. • Describe pressure-dependent and pressure-independent VVT zone controller operations .

Supplemental Material: Book

TDP No.

TDP-631 TDP-802

Cat. No. 796-056 811-10088 06-794-075

Instructor CD Cat. No.

797-056 06-797-075

Title

Rooftops, Level 1, Constant Volume Demand Control Ventilation Design Guide Controls, Level 2: DOC Networking

Instructor Information Each TDP topic is supported with a number of different items to meet the specific needs of the user. Instructor materials consist of a CD-ROM disk that includes a PowerPoint™ presentation with convenient links to all required support materials required for the topic. This always includes: slides, presenter notes, text file including work sessions and work session solutions, quiz and quiz answers. Depending upon the topic, the instructor CD may also include sound, video , spreadsheets, forms, or other material required to present a complete class. Self-study or student material consists of a text including work sessions and work session answers, and may also include forms, worksheets, calculators, etc.

Turn to the ExpertS.

Carrier Corporation Technical Training 800 644-5544 www.training .carrier.com

Form No. TDP-704A Supersedes Form No. TDP-50 and TDP-704

Cat. No. 796-069 Supersedes Cat. No. 791-464

26-Variable Volume and Temperature Systems (TDP-704).pdf

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