Natural gas combustion produces roughly 2 cubic feet of water vapor for every cubic foot of gas burned, so flue gas carries a lot of latent heat that condensing boilers recover. It also contains CO when combustion is incomplete, NOx formed at high flame temperatures, and sulfur oxides from oil. Its dew point, around 130 degrees F for natural gas, decides where condensation starts in the vent.
Flue gas is condensing in that old masonry chimney, look at the white stains on the brick, it needs a liner.
Also heard as
- products of combustion
- POC
- exhaust
- flue products
Related terms
Stack temperature
The temperature of the flue gas as it leaves the boiler, a direct measure of how much heat is going up the chimney.
Condensing boiler
A boiler designed to cool flue gas below its dew point so water vapor condenses and gives up its latent heat.
Air-free CO
Carbon monoxide in flue gas corrected to zero excess oxygen, so dilution with air cannot hide a CO problem.
Breeching
The duct that carries flue gas from a boiler's outlet to the chimney or stack.