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.
How often should I get my condensing boiler serviced?
It's important to have any gas or oil fired appliance like a condensing boiler serviced once a year. The flue will need checking to make sure it is clear, and a qualified installer should check that your boiler is running safely and efficiently for you. It's best to check with your manufacturer about service schedules, as some boilers may need servicing more than others.
How can a condensing boiler save me money on my energy bills?
Condensing boilers are more efficient than standard boilers and therefore utilise less gas during operation. Modern conventional boilers can only offer a typical efficiency rate of 78%, whereas the rate for condensing boilers is around 88% - i.e. for every £1 spent spent on gas, 88p is converted to actual heat. This adds up to significant savings on annual gas bills, which will enable you to quickly recoup the initial outlay for the boiler and benefit from the extra savings. Complete our Home Energy Survey to find out just how long payback will take in your situation.
What does the Sedbuk efficiency rating mean?
Sedbuk stands for 'seasonal efficiency of a domestic boiler in the UK' and is now given as a percentage score. This score is an estimate of efficiency when the boiler is installed under typical conditions in Britain, taking into account climate, housing conditions, occupancy patterns, and controls.
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