How do Condensing Boilers work?

While Condensing Boilers are like normal boilers in many ways, the key difference is in that they try to recover as much waste heat as possible. They do this by reclaiming heat from the waste gases your boiler produces.

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Technical information made simple

All new boilers sold in the UK must be condensing boilers which are rated ‘A’ for efficiency (At least 88% efficiency). They achieve this high efficiency by reclaiming the heat from the hot gases that all boilers produce, which a standard boiler doesn’t do.

Condensing boilers work by using a second heat exchanger to reclaim heat from hot waste gases leaving the boiler, using this heat to heat incoming cold water. This means that the boiler needs to use less fuel to heat that water to the required temperature.

As a result of this heat reclamation process, some of the water vapour in the waste gases condenses into a liquid, which drains away. The gases from the flue of a condensing boiler are typically 50-60°C compared with 120-180°C in a non-condensing boiler, which means that they have a lower propensity to rise.

This means that they need to have a shorter flue. Because of this, condensing boilers need to be situated on an outside wall, preferably on a second floor as the gases will travel less vertically than in a standard boiler. A higher location will also help the condensate drain away – you should try to connect the condensate pipe to another waste water pipe. If you cannot situate your boiler high up, you can use a pump to help the condensate drain away.

Condensing boilers do not always run in ‘condensing mode’ but because they have effective heat exchangers this doesn’t seriously affect the efficiency of the boiler.

Installation

It is important that you use a qualified installer to fit your boiler. Costs can vary considerably, so it’s advisable to get three quotes – our service provides you with three quotes from accredited local installers.

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Find out more about the Installation of Condensing Boilers here.

FAQs

What is a condensing boiler?
A condensing boiler works on the principle of recovering as much as possible of the waste heat which normally leaves through the flue of a conventional (non-condensing) boiler as water vapour. The best high efficiency condensing boilers convert more than 95% of their fuel into heat, compared to 78% for conventional types. Condensing boilers get their name because as they recapture the heat from the water vapour leaving through the flue, this water vapour condenses back to a liquid, or condensate as it is known.
If I buy a new boiler, does it have to be a condensing boiler?
Yes. Government legislation now requires all new boilers to be the condensing type unless it would be impossible to fit one, which would probably be the case if there was no way to provide suitable drainage.
How is the Sedbuk rating calculated?
The 'seasonal efficiency of a domestic boiler in the UK' (Sedbuk) rating for boilers is calculated from the results of standard laboratory tests, together with other important factors such as boiler type and the kind of fuel used. It describes the boiler's efficiency.
How reliable are condensing boilers?
Condensing boilers are reliable if well maintained and serviced regularly. A Which? survey in 2011 found that 46% of owners who bought a boiler after April 2005 have experienced some kind of fault. Not all of these faults required a repair. In fact the most frequent problem, a blocked drainage pipe, experienced by 11% of owners, is actually a problem caused by poor installation.
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