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What is a condensing boiler and how does it work?

Condensing Boilers

In 2005 it became compulsory for all new gas-fired boilers installed to be condensing, as well as have an efficiency rating of ‘A‘[137].

These regulations were introduced in a bid to decrease the CO2 emissions emitted by non-condensing boilers in the UK.

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Price Guarantee[28]

Simply answer a few multiple choice questions about your existing boiler and we’ll give you a fixed price for a new boiler including all parts and installation[28]

What is a condensing boiler?

Condensing boilers are water heating appliances run on either gas or oil, designed with the aim to improve energy efficiency by converting water vapor condensation into heat for your home. Condensing boilers have all the same elements as non-condensing boilers, but the difference is the amount of useable heat it produces from the fuel it burns.

Replacing an old G-rated non-condensing boiler with a new high-efficiency condensing boiler and improving your heating controls could save you as much as £200 a year[2]. The efficiency of a boiler is a measure of its ability to convert energy from the fuel that goes in, to heat in the water that circulates around your house.

How does a condensing boiler work?

Condensing boilers have two internal heat exchangers which means that the heat from flue gases lost to the atmosphere is decreased by recycling the heat back into the heating system. The water vapor in the flue gases can be so cool that it condenses and is removed to a waste pipe through a condensate pipe.

Unlike non-condensing boilers, condensing boilers do not need a pilot light burning constantly because they only ignite when there is a demand for heat, using an electric spark.

How do I know if my boiler is condensing?

If your boiler was installed after April 2005, it is most likely a condensing boiler due to the regulations introduced which mean that all new boilers installed, or boilers replaced after April 2005 are required to be condensing. You could check your boiler manual for reference to condensing or ‘HE‘ in the title.

Check the flue terminal which is usually sticking out through an external wall near the boiler. If it is plastic and white steam comes out of it when the boiler is on then it is likely to be a condensing boiler. Non-condensing boilers normally have a metal flue and the gases may not be visible because they‘re much hotter than those from a condensing boiler.

From a condensing boiler, there could also be a white plastic condensate pipe that leads to a drain.

You could also contact the manufacturer to query the boiler model to find out.

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[2] Source: Saving calculated when replacing G rated with A rated boiler in a semi-detached house.

[28] Exclusions apply. See