How is Combustion Efficiency calculated?     

The objective of a boiler is to burn the hydrogen contained in the fuel with oxygen from the atmosphere to produce heat. Combustion efficiency analysers exploit the fact that by knowing the fuel (and its chemical composition) and measuring the flue gas temperature and either the oxygen or carbon dioxide level the combustion efficiency of the boiler can be calculated. On some boilers the settings can then be adjusted to maximise the combustion efficiency.

In a perfect world the maximum efficiency would be achieved with 0% oxygen in the flue and the lowest flue gas temperature. In the real world allowance must be made for variations and uncertainties and so 0% oxygen is not practical. The settings on a boiler must allow for differences in fuel composition, atmospheric pressure, wind direction, boiler demands etc.

If the oxygen level is set too low and something changes the combustion process can become 'fuel rich' as there is insufficient oxygen for all the fuel to burn. This can cause high levels of CO to be generated and in the extreme enough fuel to enetr boilers flue and ignite (explode) outside the combustion chamber. Typically for a natural gas boiler oxygen readings may be in the range 3% to 5%, for an oil boiler 5% to 8% and for a coal fired boiler 8% to 10%.

The combustion efficiency of modern condensing gas boiler can theoretically be over 100% as heat is extracted from the incoming air.

 A traditional brick built coal fired boiler may only be 50% efficient.