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 (to the right) 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.
Condensing boilers can be used with gas or oil fuels.

Demonstration Projects:
Low-income housing with reduced energy costs, Houtvenne, Belgium
Poland, Rawicz: Rawicz Hospital – Use of solar technology in public buildings

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