Condensing boilers are water heaters fueled by gas. They achieve high efficiency (typically greater than 90% on the higher heating value) by condensing water vapour in the exhaust gases and so recovering its latent heat of vaporisation, which would otherwise have been wasted.

The main reason why condensing boilers are “better” than non–condensing ones is that they are more efficient. Their level of efficiency is achieved by using waste heat in flue gas to preheat cold water entering the boiler, capturing more heat from the outside rather than the inside of a room.

Condensing boilers are able to recover most of the heat that would usually be lost from the waste gases through the flue. The waste gases travel through a heat exchanger, which cools and condenses them back into a liquid known as condensate. … A good condensing boiler can achieve energy efficiency of over 90%.