Environment Watch

Condensing Boilers

It is easy to be mislead by advertisers' claims for environment advantages of their products - in this case condensing boiler. We will look at the environmental claim and see what parts are true and which parts are misleading.

The following claim was made for a condensing boiler:

If your boiler is over 10 years old, simply by replacing it with a new high efficiency condensing boiler, you could cut your gas bill and CO2 emission by 32%, and by adding the right heading controls you could bump that figure up to a dramatic 40%.

To make the figures more believable, the Energy Saving Trust was cited as the source of the percentage savings in April 2006.


A condensing gas-fired boiler is a new type of boiler which makes maximum use of the heat energy produced by the gas combustion process. Using its "super-exchanger", combining the functions of exchanger and condenser, the boiler first recovers the sensible heat from the combustion products, with an efficiency of about 12% greater than that of a traditional boiler even without condensing. If the flue gases are evacuated at this stage of combustion, they are at temperatures of 200 C to 300 C. These flue gases still contain some of the sensible heat and in particular appreciable amounts of latent heat in the form of water vapour. By routing the heating return through the bottom of the exchanger/condenser at a temperature of less than 53C, the flue gases will condense on the walls of the last rows of tubes. This condensation allied to the high performance of the exchanger results in an energy saving of up to 30% compared to traditional equipment. As the phenomenon of condensation only occurs for heating return temperatures of below 53 C, the operating efficiency of the heating installation will increase as the average annual heating return temperature decreases.

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