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.
Is a combination condensing boiler more efficient than a regular condensing boiler?
A combination (Combi) condensing boiler can be more efficient than a standard condensing boiler because it heats water as you need it. Combi boilers are most suitable for smaller homes, as they can only keep up with a certain level of demand. Non-combi boilers produce hot water and then store it in a cylinder for use as and when. This is still quite an efficient process - but if the hot water is not used immediately, then inevitably some heat will be wasted. It will generally depend on the size and amount of hot water you're likely to need. You installer will be able to advise you which type of boiler best suits your needs.
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.
Could a condensing boiler save money on my energy bill?
Which? estimates that the average efficiency of boilers in UK homes is about 70%. This means they waste up to 30% of the heat they generate in the form of hot flue gases. This suggests that installing a condensing boiler, which has an efficiency of around 88%, can reasonably achieve savings of between £50 and £200 per year depending on the size of your house - and the saving will increase as the cost of gas increases. Complete our free assessment to find out just how much you could save.
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