Definitions for "Combustion Efficiency"
(HVAC) Combustion efficiency is a measure of the heating efficiency of a boiler.  That is, it is a measure of the number of units of input energy required to produce a unit of output heat.  The combustion efficiency of a boiler is measured once the boiler is up and running (at steady state).  This makes combustion efficiency a less accurate measurement of the overall efficiency of a boiler than the AFUE which takes into account the boiler (or furnace's) efficiency as it cycles through startup and shutdown.
The effectiveness of the burner to completely burn the fuel. A well designed burner will operate with as little as 10 to 20% excess air, while converting all combustibles in the fuel to useful energy.
The amount of heat usefully available divided by the maximum amount which can be liberated by combustion; usually expressed in percentage.