A condensing combi boiler is a boiler that has a more efficient method of extracting heat from combustible fuel than a traditional oil or gas fired boiler. A huge amount of heat is wasted in a normal non-condensing gas or oil boiler in the form of hot flue gases that escape up the flue vent. This gas will be at a temperature of about 180C.

A condensing combi boiler is designed to let these gases rise as in a standard boiler but at the top the gases are redirected towards and cycled around a secondary heat exchanger, which draws heat from them, leaving them at a temperature of approximately 60C. Some condensing boilers even have dual heat exchangers, to extract even more heat from these waste gases.

This heat extraction and subsequent loss of temperature causes much of this gas to condense into water vapour, which then falls to the bottom of the flue, where it is channelled either into a pipe leading to the household soil pipe or into a separate drainage pipe that leads outside. A circulating fan then blasts the remaining waste gas up the flue and away from the property.

Compared to standard non-condensing boilers which peak at around 75% efficiency, condensing boilers can be anywhere from 83% to 92% efficient, giving them a SEDBUK rating of B or A respectively, the highest possible.