Combustion efficiency is one of those terms which are bandied about and sound as if everybody should know what is meant. The problem is that combustion efficiency is only one small part of the total efficiency of a heating system. Particularly with old systems, the radiation losses from the boiler and the poor lagging on the pipe-work can lead to losses far in excess of any stack loss or loss by incomplete combustion.
Combustion efficiency is, as the name suggests, purely a measure of the amount of energy that is lost up the stack and, perhaps, lost by unsatisfactory conversion of fuel and air to carbon dioxide and water. As such, it is a good measure of what can be gained by tuning a boiler, or better still, what has been gained by tuning a boiler, but it does not and cannot take the other factors into consideration.
It is a value readily measured by a combustion analyzer, and this makes it very useful for boiler maintenance technicians, who cannot be expected to take radiation losses into account. These losses can only be altered by changing to a newer boiler anyway.