Gas heating condensationJuly 11, 2014
When a gas fitting appliance is first ignited the flue is cold, so it absorbs heat from the flue gases, cooling them below their dew point and thus causing condensation inside the flue before the flue gases are expelled.
The condensation runs down the inside of the flue, where a small amount will be reheated and evaporated. This cycle continues until the flue heats up to normal operating temperature, then condensation should stop forming.
If the flue gas temperature is too low to begin with, or if heat loss from the flue is too great, condensation will continue to form inside the flue. If a flue joint is made the wrong way round, then condensation may attack and penetrate the sealing compound and leak through the joint.
In a natural drought flue, excessive condensation runs out of drought diverters, causing water damage. Natural drought appliances usually have fixed heat inputs. If condensation persists after the flue has warmed up to operating temperature, you must insulate the flue to reduce heat loss.
In forced drought flues, excessive condensation runs back into the combustion chamber and may interfere with combustion. Insulating the flue will reduce heat loss, but it is also possible to raise the flue gas temperature above dew point by burning more fuel gas. This adjustment must not cause incomplete combustion or poor thermal efficiency; nor must the appliance be made too expensive to run. Good to keep in mind when your heating your home. A simple gas service can be done to prepare you for the long Winter ahead.