Excess Air

Excess air is the combustion air supplied beyond the exact amount needed to burn the fuel completely. A little excess ensures complete combustion, but too much carries useful heat out the stack and wastes fuel.

In practice burners need slightly more air than stoichiometric to avoid incomplete combustion and carbon monoxide. The optimum excess air is the smallest amount that still gives clean, complete burning, typically expressed as a percentage or measured indirectly through flue-gas oxygen. Tuning a burner to minimum safe excess air is one of the highest-return combustion efficiency measures, since every extra unit of air must be heated to flue temperature and then discarded.

Related terms

Stoichiometric Combustion · Flue Gas Loss · Boiler Efficiency