What is a Condensing Boiler?

A condensing boiler is a high efficiency modern boiler that incorporates an extra heat exchanger so that the hot exhaust gases lose much of their energy to pre-heat the water in the boiler system. When working at peak efficiency, the water vapour produced in the combustion process condenses back into liquid form releasing the latent heat of vaporisation. A side effect is that this water, known as condensate, which is usually acidic, has to be piped away to a drain or soakaway.

The photo (below) shows a cutaway combination condensing boiler. It is mounted on a wall and the exhaust gases will rise through the plastic flue in the top left corner. Hot water is provided by a small storage tank on the right: the tank (which is covered by insulating foam) has been cut open to show the tightly wound quick refresh coil inside it. At the bottom of the photo are a number of pipes going into the boiler. One carries the gas for the burner and there are two (in and out) for the central heating system. The plastic pipe on the right carries the condensed water vapour produced by burning the gas. This water contains dissolved oxides of sulphur and nitrogen, making it slightly acidic.

Do Condensing Boilers Work?

Ian Byrne, the Foundation's Deputy Director writes: "YES! Although not many homes in Britain have condensing boilers, over half the new boilers in the Netherlands have been of this type for a number of years. The National Energy Foundation carried out extensive monitoring work on new homes built in Milton Keynes in the late 1980s, and is was found that operating efficiencies of 95% or better could be obtained in normal use. I have had a condensing boiler in my own home for 15 years; it works very well and my heating bills are lower than my neighbours in similar homes with old-fashioned boilers (although most have installed condensing boilers themselves since this page was first written in 1996)."