Wood heating
Chimney Design
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Presentation Summary Overview of Requirements – Functions – Definitions, Classifications & Designations – System Requirements – Legal Requirements
Introduction to – General Principles, Flue Dynamics – Chimney Design – Chimney Construction, Materials – Draught Influence & Control Methods
Examples of Chimneys. Domestic implies Definition.
≤
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50kW appliance output ref Doc J.
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Solid Biofuel Domestic Heating Chimneys
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Examples of Chimneys. Domestic implies Definition.
≤
50kW appliance output ref Doc J.
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FUNCTIONS Primarily 1) To discharge the products of combustion into the enviroment without causing inconvenience 2) To safely discharge products of combustion through the structure of the building 3) To provide assistance or total force for combustion air input
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References; 1) Building Regs Approved Doc J:2002, Requirements J2, J3 2) 45 kW and above - BS 5854:1980, Requirement 5.1 3) European Standard BS EN 1443:1999
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DEFINITIONS 1) “ Products of combustion ; products resulting from the combustion of fuel (Gaseous, liquid & solid constituents)” a) “Flue gas; gaseous portion of products of combustion conveyed in a flue” b) Liquid; condensates, (possible rain ingress) c) Solid; soot, smuts, fly-ash 5
Reference; European Standard BS EN 1443:1999, Chimneys- General requirements
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DEFINITIONS a) System Chimneys- installed in accordance, using a combination of compatible components obtained or specified from one manufacturing source, with product responsibility for the whole chimney; or
b) Custom Made Chimneys- installed as above using compatible components from more than one source • • •
EN Classifications & Designations Apply Installers Product Liability Insurance ? Designers professional indemnity insuranc e applies 6
Ref; BS EN 1443:1999 Chimneys general requirements
BS EN 12391 Metal chimneys - Execution
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Solid Biofuel Domestic Heating
Chimneys
DEFINITIONS 1 2 3 4 5 6 7 8
Chimney Flue Flue Liner Thermal Insulation Outer wall Enclosure or cladding Chimney section Multi-wall chimney (eg; Flue Block) 9 Chimney fitting 10 Connecting flue pipe 11 Heating appliance Terminal
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Reference; EN 1443:1999
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Solid Biofuel Domestic Heating
Chimneys
CLASSIFICATION Chimneys are classified in accordance with the following performance characteristics:
G
Minimum specification for wood fuel: T400 N2 D 3 Gxx
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Solid Biofuel Domestic Heating
General
Chimneys
Requirements
1.
Discharge at sufficient height for convenient dispersal of products of combustion, including any airborne solids!
2.
Discharge total volume of flue gases,
3.
Facilitate removal of solid residues eg; fly-ash
4.
Allow for dispersal of any liquid products eg; condensates
5.
Structural Integrity and Stability
6.
Material Durability
7.
Protection of Building eg; Risk of Fire
8.
Protection of Occupants eg; Risk of CO poisoning
p.o.c + entrained air
eg; loadbearing
eg; resistance to temperature & corrosion
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Solid Biofuel Domestic Heating
Chimneys
Legal Requirements
•
Building Control Approval - Prior
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Planning Permission - Local Authorities,
application to & inspection by local authority OR installation by ‘Competent Person’ – eg H.E.T.A.S followed by notification National Parks, Conservation areas, listed buildings etc.
• •
Clean Air Act - Smoke Control Areas Environmental Health – Large non-domestic systems – Neighbour complaints 10
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Solid Biofuel Domestic Heating
Building Regulations
Chimneys ‘Approved Document J’ - April 2002 England & Wales The Requirements J1
Air Supply
J2
Discharge of products of combustion
J3
Protection of Building
J4
Provision of Information 11
Different Regulations and guidance in SCOTLAND and NI.
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Solid Biofuel Domestic Heating
Chimneys
General Guidance J1 Air Supply
Applies to all fuels 12
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Solid Biofuel Domestic Heating
Chimneys
Specific Guidance J1 Air Supply Section 2 Applies to solid fuels
Closed appliance – no draught stabiliser 550mm² of permanently open free area Of ventilation for every kW above 5.0 kW Closed appliance – with draught stabiliser 300mm² for the first 5 kW and 850mm² of permanently open free area of ventilation for every kW above 5.0kW 13
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Solid Biofuel Domestic Heatin g
Chimneys
J2 Discharge of the products of combustion
Specific Guidance
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Lateral Support Stays usually required 1.5m above roof line
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Solid Biofuel Domestic Heating
Chimneys
J3 Protection of Building
Specific Guidance
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Ventilated Firestops sometimes required. No combustible surrounding surfaces to exceed 100C in any test conditions.
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Solid Biofuel Domestic Heating
Chimneys
J4 Provision of information
Specific Guidance
Data Plate
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Draught types •
Natural draught Usually gives Negative pressure within flue
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Induced draught Usually gives Negative pressure within flue
•
‘Forced’ draught by blower fan which usually gives combustion air, can cause Positive pressure within the flue
•
Measurement by ‘ draught gauge’ to achieve manufactures target
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Definitions ref Doc J
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Flue draught Flue Draught also important in ‘naturally draughted’ appliances to control fuel/air mix of combustion. Influenced by •
Properties & Design of chimney & by the action of wind amongst other factors……
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Solid Biofuel Domestic Heating
Natural draught
Chimneys ‘Stack Effect’ , Displacement Heated flue gas expands, becomes less dense & is ‘pushed’ up the chimney by a colder, denser ‘column’ of surrounding air of equal volume i.e. Buoyancy The greater the temp. difference the greater the pressure difference or ‘draught ’ The taller the ‘columns’ the greater the pressure differential & combined with the above the greater the ‘natural draught ’
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Chimney Design Strategy Combine energy conservation with low resistance – Route through structure, • Tend to the vertical • Limits to offset bends & inclined runs • Limits to horizontal runs – Cleaning Access provision (whole chimney) • To Debris collection space •
For Rods, Brushes, Vacuum hoses
– Terminal – Additional Accessories e.g.; • Condensate or rain drains • Draught Control devices – Flue Draught Stabilisers – Special Cowls 20
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Chimney Design Sizing of Flue – Height minimum recommended 4.5m (ADJ) but must also be Sufficient for dispersal – Be discharged at a required rate dependant on combustion of fuel – Diameter of Connecting Flue Pipe may differ to remainder of chimney Remember – Adopt Guidance from Building Regs – Adhere to system manufacturers’ recommendations if require larger than regulations state – Calculations can be applied to provide smaller flues if carried out in a recognised method 21
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Chimney Construction Considerations include; – System & Material choice (matching appliance and chimney designations) Lateral & Vertical support – Fire protection – Weathering details – Lightning conduction & earthing ! – Access, scaffolding – Materials handling 22
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Draught Control •
By design of terminal ‘cowl’
•
This may be possible if wind strongly influences the draught.
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Flue Draught Stabilisers • •
Additional ventilation required Types, Sizes Positioning considerations – Same room as combustion air – Distance from appliance – Distance from combustibles
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Flue materials • Clay Liners, Concrete • Pumice Liners & Modular Systems • Metal; Cast Iron 3 mm. , Mild steel 1 mm. Stainless steel Grade 316 Vitreous enamelled low carbon steel 25
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Maintenance • Sweeping
at least 2 x each heating season (6 x in Austria as instructed by the sweep in Germany) • Visual Inspection Testing; smoke, clearance • Repairs Lining, re-pointing or replacement components 26
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Sizing of Flue – Methods • BS 5854 • BS EN 13384 • CIBSE Design Guide B (The most common method) – All requires complex calculations – Spreadsheet analysis easier •
Designing a chimney to provide the correct amount of natural draught involves a number design factors, many of which require trial-and-error reiterative methods
•
As a "first guess" approximations can be used to estimate the natural draught/draft flow rate by assuming that the molecular mass (i.e., molecular weight) of the flue gas and the external air are equal and that the frictional pressure and heat losses are negligible 27
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Simple calculation-
∆P
• Pressure difference ( ∆P ) is the driving force for the stack effect and it can be approximately calculated with the equation below • For flue gas stacks and chimneys, where air is on the outside and combustion flue gases are on the inside, the equations will only provide an approximation of the height (h) of the chimney. Where: ∆P= available pressure difference in Pa C= 0.0342 a= atmospheric pressure, in Pa h= height, in m To= absolute outside temperature, in K Ti= absolute flue gas temperature, in K 28
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Simple calculation- Q •Draft or draught flow rate ( Q) which is induced by the stack effect can be calculated with the equation below •For flue gas stacks or chimneys, where air is on the outside and combustion flue gases are on the inside, the equation will only provide an approximation. Also, the cross-sectional flow area ( A) and is the height ( h) of the flue gas stack or chimney. Where: Q= stack effect draft/draught flow rate, m³/s A= flow area, m² C= discharge coefficient (usually taken to be from 0.65 to 0.70) g= gravitational acceleration, 9.807 m/s² h= height, m Ti= average inside temperature, KT To= outside air temperature, K
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Full method –Appliance parameters • Appliance maximum and minimum rated outputs • Appliance efficiency (gross) • Flue outlet temperatures at both maximum and minimum outputs • Minimum and maximum draught required at the boiler outlet.
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Full method– Fuel parameters • Fuel type (Gas, oil type of solid fuel) • Calorific value • Required excess oxygen level • Required optimum oxygen consumption • Fuel analysis including carbon, hydrogen and sulphur content. 31
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Full method– Chimney parameters • Proposed inner surface material • Proposed height • U values for heat loss • Proposed diameter • Proposed eflux velocity
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Full method– Environmental factors • Flue terminal location • Ambient temperatures (max and min expected) • Topography
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