Combustion efficiency as defined in flue gas analysis standards is simply reduced by the stack loss. This means no account of radiation or other losses is taken when calculating combustion efficiency. Clearly, this is not an accurate picture of the total efficiency, but combustion efficiency is a very convenient term to use when comparing burner systems. It will particularly provide a measure of the factors easily changed when tuning a burner. Changes in combustion efficiency will generally give a good suggestion of the state of the burner, with drops meaning that maintenance of some sort is due. Sometimes a combustion efficiency level will be included in local standards, but this is not really a useful contribution. It is far more productive to maximise the efficiency whilst remaining within the limits for carbon monoxide. This value of combustion efficiency will not take any account of the condensation effects from condensing burners either.
Combustion efficiency can also be altered and improved by a number of simple methods. Air preheaters are a particularly simple method for large burners. Fuel preheating to improve mixing and atomisation is also helpful.