What is a condensing boiler?

Condensing boilers get their name because they condense periodically. In other words, they extract heat from the exhaust gases that would otherwise escape through the flue - turning water vapour from the gas back into liquid water or condensate.

A condensing boiler captures much more usable heat from its fuel than a non-condensing boiler. Its high operating efficiency is made possible by the design of the condensing boiler's larger - or sometimes dual - heat exchanger.

The heat exchanger makes sure that as much heat as possible is transferred from the boiler's burner - and as little as possible lost in gases through the flue.

Viessmann graphic (courtesy of Viessmann, Telford), explaining the process: