Excess Air from O2 Calculator

One reading tells you a lot about combustion efficiency

Rather than measuring fuel flow and air flow separately, technicians can estimate excess combustion air directly from a single flue gas oxygen percentage reading - a much faster field measurement using a portable combustion analyzer.

Worked example

For a flue gas oxygen reading of 3.5%:

Excess Air = 3.5 / (20.9 - 3.5) x 100 = 20.11%

Flue Gas O2	Approximate Excess Air
2%	~10.6%
3.5%	~20.1%
5%	~31.4%

Well-tuned combustion equipment commonly targets 10-20% excess air - enough to ensure complete combustion without wasting fuel heating unnecessary excess air pulled through the system.