What is special about a condensing boiler?

Some modern boilers are now described as being condensing boilers. The combustion efficiency calculation must be modified to properly reflect the efficiency of the combustion process within these boilers.

The practical difference is that a condensing boiler utilises an additional heat exchanger just before it exhausts the flue gases. This extracts additional heat from the flue gas and further reduces energy losses.

Under certain circumstances this can lead to net condensing combustion efficiencies of greater than 100%.

Typically for a natural gas fuelled boiler the temperature of the flue gas being exhausted needs to be less than 58 °C for the condensation process to recover additional energy.

Above 58 °C the normal combustion efficiency calculation operates.