Excess Air
The amount of air supplied to a burner beyond what is strictly required for complete combustion of the gas.
What is excess air?
Burning a given volume of natural gas completely requires, in theory, a precise quantity of air — the stoichiometric air requirement. In a real appliance, gas and air never mix perfectly, so the burner is fed slightly more air than that theoretical minimum. That margin is called excess air, and it is expressed as a percentage of the theoretical air.
Why is the setting a trade-off?
Too little air and part of the fuel does not burn fully: combustion becomes incomplete, producing carbon monoxide and deposits. Too much air and the burner is heating nitrogen and oxygen that leave through the stack almost immediately, taking with them heat that should have gone to the building or the process. Tuning a burner means staying inside the useful band between those two failure modes — the band the appliance manufacturer documents.
How is excess air tracked in operation?
It is one of the parameters a combustion analysis turns into hard numbers: the oxygen content of the flue gases, read with a calibrated instrument, is used to estimate excess air under representative operating conditions. The result is always read together with residual CO, flue gas temperature and draft. Excess air also shapes flame temperature and therefore the formation of NOx. Any adjustment to the air-fuel ratio is work for a qualified technician, following the manufacturer’s instructions and applicable codes.