Definition: A condensing boiler recovers heat from its own flue gases that older boilers waste, reaching about 89% efficiency versus 60-75% for non-condensing models. All new UK gas boilers must be condensing.

How it works

A larger heat exchanger cools flue gases until the water vapour condenses, releasing latent heat back into your heating water. The visible plume from the flue is a sign it’s working.

Upgrade savings

Replacing a G-rated boiler with an A-rated condensing model saves around £300+ a year in a typical home — more in larger or older properties.

Estimate your numbers: try the free savings calculator or browse all upgrade guides.