COMMERCIAL LOAD ESTIMATING
Load Estimating Level 3: Block and Zone Loads
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 advancedlevel design course. Advanced-level modules assume prerequisite knowledge and do not review basic concepts.
The block and zone load portion of commercial load estimating takes the designer through the process of making a practical estimate of the amount of heating and/or cooling energy needed to condition a building. Block and zone load estimates provide the data necessary to select heating and cooling equipment that can condition the spaces within a building. Using the outputs for the building block and zone load estimates, the HVAC system equipment selections can be made to complete the design. Along with psychrometrics, load estimating establishes the foundation upon which HVAC system design and operation occur. Load Estimating, Level 3: Block and Zone Loads is the third in a four-part series on load estimating. It is preceded by overview and fundamentals topics, and followed by a TDP that reviews refinements required for preparing system-based design load estimates.
© 2006 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.
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Table of Contents Introduction............................................................................................................................................... 11 Load Estimating Process Overview ........................................................................................... 33 Step 1 – Gather Building Information ................................................................................... 33 Step 2 – Preliminary Equipment Zoning ............................................................................... 33 Step 3 – Input Space Data ...................................................................................................... 33 Step 4 – Control Zoning......................................................................................................... 33 Step 5 – Input HVAC System Parameters ............................................................................. 33 Step 6 – Run Load Calculations............................................................................................. 44 Step 7 – Interpret Results, Revise if Necessary ..................................................................... 44 Fundamental Terminology ......................................................................................................... 44 Step 1 – Gather Building Information...................................................................................................... 77 Example Building – Chicago Office Building ........................................................................... 77 Building Take-Off...................................................................................................................... 99 Building Take-Off...................................................................................................................... 99 Step 2 – Preliminary Equipment Zoning .............................................................................................1010 Zoning Methods ..................................................................................................................... 1010 Perimeter / Core Zoning Method ....................................................................................... 1010 Exposure Zoning Method .................................................................................................. 1111 Usage Zoning Method ....................................................................................................... 1212 Step 3 –Input Space Data .....................................................................................................................1212 Outdoor Design Conditions ................................................................................................... 1313 Sources ............................................................................................................................... 1414 Dry Bulb / Mean Coincident Wet Bulb ............................................................................. 1414 Wet Bulb / Mean Coincident Dry Bulb ............................................................................. 1414 Dew Point / Mean Coincident Dry Bulb and Humidity Ratio ........................................... 1515 Humidity Ratio .................................................................................................................. 1515 Mean Daily Range ............................................................................................................. 1515 Prevailing Wind Speed ...................................................................................................... 1515 Heating Dry Bulb ............................................................................................................... 1515 Latitude and Longitude ...................................................................................................... 1616 Elevation ............................................................................................................................ 1616 Thermostat Set Points ............................................................................................................ 1616 Lighting .................................................................................................................................. 1717 People..................................................................................................................................... 1818 Electric Equipment ................................................................................................................ 1919 Plug Load Usage Diversity ................................................................................................ 1919 Load Factors ...................................................................................................................... 2020 Electric Motors ...................................................................................................................... 2020 Partitions ................................................................................................................................ 2020 Internal Load Scheduling ....................................................................................................... 2121 Diversity Factors ................................................................................................................ 2121 Hourly Scheduling ............................................................................................................. 2222 Thermostat Scheduling ...................................................................................................... 2222 Latent Sources........................................................................................................................ 2222 Chicago Office Building Input Review ................................................................................. 2323 Step 4 – Control Zoning .......................................................................................................................2323 Zoning for Comfort ................................................................................................................ 2323 Balancing Comfort and Economics ................................................................................... 2424 Step 5 – Input HVAC System Parameters ...........................................................................................2525 System Types ......................................................................................................................... 2525 System Loads ......................................................................................................................... 2626
Ventilation Versus Infiltration ............................................................................................... 2727 Direct Exhaust ........................................................................................................................ 2727 Step 6 – Run Load Calculations ........................................................................................................... 2929 Review Inputs and File Take-Off Sheets ............................................................................... 2929 Select Reports for Viewing and Printing ............................................................................... 2929 Air System Sizing Summary .............................................................................................. 3030 System Design Load Summary .......................................................................................... 3030 Zone Design Load Summary ............................................................................................. 3030 System Psychrometric Graph ............................................................................................. 3131 Ventilation Sizing Summary .............................................................................................. 3131 Hourly Air System Loads .................................................................................................. 3131 Hourly Zone Loads ............................................................................................................ 3131 System Psychrometrics Table ............................................................................................ 3131 Archive Project to a Back-Up Location ................................................................................. 3131 Step 7 – Interpret Results, Revise If Needed ....................................................................................... 3232 Design Check Figure Review ................................................................................................ 3232 Peak Load Times.................................................................................................................... 3434 System Load Sensitivity......................................................................................................... 3535 Resulting Relative Humidity .................................................................................................. 3535 Creeping Safety Factor........................................................................................................... 3636 Plotting Outputs on the Psychrometric Chart ...................................................................................... 3737 State Points............................................................................................................................. 3838 Coil Process Line ................................................................................................................... 3838 Room Sensible Heat Factor (RSHF) Line.............................................................................. 3939 Gains and Losses Off of the RSHF Line................................................................................ 3939 Load Estimate Outputs for Equipment Selection ................................................................................ 4040 Summary ............................................................................................................................................... 4040 Work Session 1 ..................................................................................................................................... 4141 Work Session 2 ..................................................................................................................................... 4343 Work Session 1 Answers...................................................................................................................... 4545 Work Session 2 Answers...................................................................................................................... 4646 Appendix A – Terms List ..................................................................................................................... 4747 Appendix B – Carrier’s Block Load 4.0 Space Input Sheets .............................................................. 4848 Appendix C – Carrier’s E-20A and E-10A Manual Estimating Forms .............................................. 5151 Appendix D –Thermal Resistances of Plane Air Spaces .................................................................... 5353 Appendix E – Typical Thermal Properties of Common Building and Insulating Materials ............. 5454 Appendix F – Table of Design Conditions .......................................................................................... 5858 Appendix G – HVAC Design Check Figures ...................................................................................... 6262 Appendix G – HVAC Design Check Figures ...................................................................................... 6363 Appendix H – Chicago Weather Reports ............................................................................................ 6565 Appendix I – Chicago Office Building Load Estimate Inputs and Outputs ....................................... 6868 References ............................................................................................................................................ 7979 Glossary ................................................................................................................................................ 8080
LOAD ESTIMATING, LEVEL 3: BLOCK AND ZONE LOADS
Introduction This Technical Development Program (TDP) training module is the third in a four-part series covering Commercial HVAC Load Estimating. While the first module mostly covered load components and theory, this TDP concentrates on the actual steps necessary to perform a load estimate. Whether you are using a software program or doing a quick manual load calculation, many of the preliminary steps are similar. Figure 1 shows a portion of a manual load calculation form.
Figure 1 Manual Load Estimate Form
Figure 2 shows inputs and outputs of a computer software load-estimating program. This module focuses primarily on computer-based load estimating methods. The manual method using Carrier’s Cooling Load Estimate Form E-20A and Heating Load Estimate Form E-10A, is presented in detail in the System Design Manual, Book One and the supplemental User’s Guide, Load Estimating Using Load Factors and Equivalent Temperatures, T200-90.
Figure 2 Load Estimating Software Inputs and Outputs
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LOAD ESTIMATING, LEVEL 3: BLOCK AND ZONE LOADS
Once the building orientation and envelope design have been finalized by the architect, the HVAC designer can begin to compile the detailed information necessary to perform a load estimate for equipment sizing and selection. This generally occurs during the design development (DD) phase of the project. The building design phases are shown in Figure 3. This information includes, but is not limited to: local design weather conditions; wall, roof, and glass material properties; solar exposures; shading; and internal load components such as Figure 3 lights, people and electrical equip- Building Design Phases ment (plug loads). At the earlier schematic stage, rough loads are computed to help eliminate certain HVAC (heating, ventilation and air conditioning) systems from further consideration. One might ask, “Why are we thinking about system types now, when we have not even calculated detailed load estimates yet?” This is because system selection and load estimating are not done in a vacuum. Loads impact system selection and system types affect final load results. For example, let’s use the well-established 400 ft2 per ton design check figure for an office building. A 20-story high-rise with 10,000 sq ft per floor could be “ball-park” sized at 500 tons. 20 stories 10,000ft 2 / floor 400ft 2 / ton 500 tons This means that it is unlikely that packaged equipment (rooftop units or split-systems) will be used, due to the large installed capacity and distance to the roof equipment location. Instead, a chilled water system would be more appropriate for this application. Likewise, a 10,000 sq ft single story office (10,000 400 = 25 tons) would likely not utilize a chiller, but rather would use a rooftop unit or DX (direct expansion) split-system.
System Information Knowing what type of system is likely to go into the building helps determine system load components when running DD load estimates.
This TDP will explain the procedures and steps necessary to gather the data needed to calculate load estimates using computer software. We will discuss the different ways to zone a building that yield the sizing results needed to do proper equipment selections. We will then interpret outputs from computerized load calculations, and see how they affect equipment selection. This introduction section contains a brief overview of the load estimating process, which is divided into seven distinct steps. Subsequent sections contain expanded details about each of the seven steps involved, along with appropriate examples to further explain the effort involved.
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LOAD ESTIMATING, LEVEL 3: BLOCK AND ZONE LOADS
Load Estimating Process Overview The process of load estimating is not always linear, however, as a general rule, the following steps are necessary to perform a load estimate.
Step 1 – Gather Building Information This step includes performing a building take-off and gathering all physical data about the building structure, layout, function, occupancy, and usage schedules. This allows proper identification of all load components and facilitates the sizing and layout of terminal units and ductwork.
Step 2 – Preliminary Equipment Zoning The building floor plan is divided into individual areas or spaces for load analysis purposes. These early zones are rough areas that are mostly based on air handler coverage. Each piece of equipment needs to be sized, based on the demands of the zone heating and cooling loads.
Step 3 – Input Space Data The space data is needed for the software to run the load estimating calculations. These include all building envelope properties (walls, windows, and roofs), internal and external heat gains, occupancy counts, and operating schedules.
Step 4 – Control Zoning At this step, the designer combines spaces into comfort control zones (areas with similar heating and cooling load characteristics). Additional system decisions will be affected. For instance, if you know that you are going to use a multiple-zone system served by packaged rooftop equipment, you will eventually need to decide how to achieve zone-level control from a single packaged unit. This could include the use of either VAV (variable air volume) terminals or a damper control system like a VVT® (variable volume and temperature) system.
Step 5 – Input HVAC System Parameters During preliminary design, system selection decisions should be far enough along to permit meaningful load estimating input. For instance, a high-rise building will likely utilize all-air or all-water central systems, while a small restaurant or retail store would likely use small packagedtype equipment. Even at this early stage, additional system decisions may be made. For instance, if you know that you are going to use zone-level equipment, like PTACs or unit ventilators, the ventilation air design decisions are different than when using central systems using mixing boxes. Similarly, there would be no ceiling plenum effects or duct heat transfer.
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LOAD ESTIMATING, LEVEL 3: BLOCK AND ZONE LOADS
Step 6 – Run Load Calculations Hopefully, as part of Step 3, blank input forms were used to compile your design load inputs. Taking the extra time required to write down the input values allows you to check and verify the data. It also offers the added advantage of creating a paper trail for others to follow should another designer have to pick up where you left off and complete the design. For this step, the run parameters are set and the computer is used to complete the run load calculations.
Step 7 – Interpret Results, Revise if Necessary Using HVAC design check figures and rules-of-thumb sizing guidelines allow you to verify that calculation results are within a reasonable tolerance level. This includes a complete psychrometric analysis to identify all state points and performance lines in the air system. Load estimate outputs are used to select most of the HVAC equipment, therefore, they must be accurate and realistic. For example, calculated results requiring very cold supply air or very warm return air temperatures are red flags indicating a possible humidity control problem or software input error. System modeling concepts can help pin down these problems and help the user make necessary corrections. Consult TDP-303, Load Estimating, Level 4: HVAC System-Based Design, for details in this application area of load estimating. Each of these steps is covered in detail in the sections that follow, but first we will present fundamental terminology and zoning concepts.
Fundamental Terminology Load estimates are the summation of heat transfer elements into (gains) or out of (losses) the spaces of a building. Each heat transfer element is called a load component. Load components represent sensible and latent heat transfers (in Btuh) from many different sources. All load components can be assembled into one of three basic groups: external space loads, internal space loads, and system loads. See Figure 4. Each group has been covered in detail in TDP-301, Load Estimating, Level 2: Fundamentals. Appendix A contains a terms list for the abbreviations found in Figure 4.
Figure 4 Cooling Load Components
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