What is the difference between combustion efficiency and boiler efficiency?

Combustion Efficiency v Boiler Efficiency

A combustion or flue gas analyser is used to measure the efficiency of the combustion process within a boiler.

This is not the same as the boiler efficiency as it does not take into account, for example, the heat losses from the case of the boiler.

So generally the efficiency stated on the rating plate of the boiler will always be lower than the measured efficiency of combustion.