The excess air for combustion such as that in a boiler contributes only to an increase in heat loss with the exhaust gas.
The degree of excess air is expressed by air ratio. The air ratio at 1 (one) represents the theoretical combustion.
The air ratio is expressed by 21/(21-O2%) where O2 is oxygen content in the exhaust gas.
The Criteria of the Law for Rational Use of Energy set both standard and target values of air ratios for boilers and industrial furnaces.