If the % CO2 content is correct (or

If the % CO2 content is correct (or the % O2 content low) and the gases are in a non-smoky condition then the combustion of the fuel is correct. With correct %CO2 content the % excess air required for combustion will be low and this results in improved boiler effciency since less heat is taken from the burning fuel by the small amount of excess air.If the excess air supply is increased then the % CO2 content of the gases will fall.