Factors for Effective Flueing
A properly functioning flue will:
- safely, reliably and effectively convey combustion products to the outside environment,
- establish appropriate draught quickly from cold starting conditions,
- minimise spillage of flue gases (products of combustion) into the room (or space where the appliance is located),
- incorporate components and design to minimise the effect of adverse draught conditions,
- avoid hazards to the structure or its contents from excessive exposure to heat, and from points where the flue system penetrates the building envelope,
- avoid (or at least minimize) hazards of leakage of combustion products (flue gases) from the flue, and of the build-up of condensation in the flue, and
- incorporate an effective termination point.
Consequences of poor flue performance include:
- Spillage of combustion products into the living environment, leading to incomplete combustion and un-burnt gases,
- Flame lift,
- Flame lightback – for certain types of burner, and
- Inefficiency (too rapid loss of heat).
Effective flueing is an important aspect of good installation practice. It brings together a number of important factors in both the design and installation of flue systems, including:
- appliance design, ventilation and location,
- internal and external environments, and
- design of flue systems, including draught diverters and terminals,
Appliance ─ design ventilation and location
Design and installation of flue systems has to consider specific factors related to the type of appliance and its design and configuration, energy consumption, and location as well as any specific manufacturer's instructions.
In addition, the location, design and configuration of the flue needs to be related to ventilation and other mechanical and physical aspects of the building.
This includes ventilation requirements for the appliance, any other combustion appliances, and for users of the space in which the appliance is located, as well as the ventilation available and any mechanical ventilation that may be present, whether or not it is associated with the appliance.
Appliance design factors that allow them to operate safely and efficiently include:
- bringing gas and air into the appliance in adequate quantities and pressures, given the type of gas used,
- means of mixing gas and air and transporting the mixture to all burner ports, and
- burner design to ensure good flame characteristics ─ flame shape and colour, flame retention (avoiding lift and lightback), avoiding flame chilling (eg through impingement on cold surfaces).
Appliances that are designed to generate radiant heat from flames (eg natural draught flame effect heaters) are more prone to producing free carbon and exhibit poor combustion. This can lead to sooting, carbon monoxide (CO) and other products of incomplete combustion. Therefore flue systems for these types of appliance are more critical.
External environment
This includes the prevailing local climate variations, and the relationship to the structure that houses it, and to other structures. In some cases, vegetation or natural features, such as proximity to shelter belt or location in a gully, may have an effect.
Local knowledge may be useful, for instance in identifying prevailing wind patterns.
Internal environment
Flueing needs to relate to the ventilation and other mechanical and physical aspects of the building. This includes:
- the space in which the appliance is situated and its use,
- the availability and amount of permanent (fixed) ventilation,
- adventitious ventilation and tightness of the structure,
- intermittent ventilation,
- pressure patterns in the installation environment, including the effect of any mechanical ventilation in the structure, even in other rooms, eg range hoods and extractor fans that can cause negative pressure and upset natural draught flues,
- potential adverse climatic conditions,
- other combustion appliances, and not just gas appliances, as these will create draught (air movement) and consume oxygen,
- atmospheric contaminants – dust, spray, chemicals, eg hairdressers and drycleaners, and
- the effect of change in orientation of components within the appliance or flue system.4
Obviously in many situations these factors may have little effect individually but the combined effect of these factors can be significant in critical situations.
Draught Diverters5
The purpose of the draught diverter (DD) is to ensure an appropriate pressure around the flame. This is achieved by diverting draught caused by changes in pressure within the internal environment or outside the building. It dampens the effect of any fluctuations in the air pressure inside or outside the building on the combustion chamber - see Figure A.
Figure A: Typical draught diverter and flue arrangements
In designing and installing the flue system the aim is to utilise the difference in temperature to create a pressure differential. This induces a flow of combustion products and free air through the flue, which draws cooler fresh air for combustion into the appliance. It also protects the flame. The effect of creating a pressure differential is often referred to by the observation that hot air is lighter than air and therefore rises.
The two forces that act through a draught diverter are downdraught pressure and lift or updraught presusure. In both cases the draught diverter acts as a bypass so that such forces are in balance and do not excessively act through the combustion chamber, and hence adversely affect combustion.
It is also important to avoid pushing the flame down by downdraught, i.e. excessive negative pressure, which can lead to spillage of combustion products into the room. In extreme cases it can lead to damage to appliance controls.
To ensure proper operation of the draught diverter there should be sufficient clearance around the diverter to ensure unrestricted air flow. In addition, it is important that the draught diverter is in the same space, and shares the same atmospheric conditions, as the appliance.
Flue design
It is important to ensure adequate temperature differentials within the flue to allow appropriate draught and to minimize the potential for condensation. It is also important to protect combustible surfaces from heat (temperature) hazards. These objectives are achieved by insulation using double skinning, clearances (air gaps) or insulating materials, and by height of flue.
Leak-proofing to ensure the integrity of the flue, the flue inlet and flue connections is also important. Seals have to be secure as any leaks or loose connections decrease the efficiency and effectiveness of the flue.
Flue dampers can provide for increased efficiency of some appliances. However, they should be avoided unless the maximum area closed would not lead to adverse operation of the appliance or to spillage of combustion products.
Manufacturers instructions must be consulted if a flue damper is contemplated.
Flue sizing6
Flues should be designed to ensure adequate draught under all operating conditions and have minimal restrictions throughout the system. In particular:
- there should be sufficient vertical rise to rapidly establish flue operation,
- the effective cross-section should never be less than the outlet cross-section of the appliance flue outlet, taking into account any offsets between transition piece inlet and outlet, and the size of the outlet,
- changes in direction should be minimized and should be even (ie not sudden)
- shape transition (ie rectangular to circular, elbows and horizontal sections, etc.) must not restrict the effective cross-section and hence the flow of combustion products, to minimise restrictions, see Figure B, and
- there should be a maximum vertical rise before any horizontal section.
Figure B: Transition
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