Principles of Flueing
In a gas appliance, after the flame has generated heat, for efficiency of heat transfer, the source of heat should be in contact with the medium to which the heat is applied for as long as possible to ensure efficient convective and radiant heat transfer.
To achieve this means having a significant resistance to flow of gas and combustion air within the appliance. In addition, draught is required to generate flow of combustion products. Resistance also means potential for spillage, in particular under cold conditions while the appliance and flue system are warming up and draught is being established.
Factors influencing good flueing include:
- establishing draught via temperature differential along the flue, so that the more buoyant lower density combustion products move upwards along the flue,
- flue height (vertical rise) and cross-section (diameter),
- minimum temperature loss along the flue to ensure effective draught, and
- minimal resistance to flow (minimising restrictions).
Factors that create resistance to flow, and therefore risk of initial spillage, include:
- inadequate height or equivalent cross-section,
- shape (cross-section) and shape transition (transition pieces),
- obstructions – including dampers and offsets,
- change of direction – especially sudden changes,
- multiple flues – causing back pressure due to convergence of flow), and
- horizontal sections of flue.3
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