What is a condensing boiler?

What is a condensing boiler?
A combi condensing boiler in a kitchenA combi condensing boiler in a kitchen

If you're researching boilers with the intention of having a new one installed in your home, it can be quite off-putting when you read all the jargon and specialised terms that seem to permeate the heating industry. A common question people have is whether or not there's a difference between a combi or a condensing boiler and it can sometimes be difficult to get a straight answer.

The simple answer is that all modern boilers installed in homes today are condensing boilers. If you are buying a new boiler then it will automatically be a condensing boiler. New installations of non-condensing boilers were outlawed in 2005. There are still many fully functional, non-condensing boilers currently in homes across the UK, but once they stop working they will need to be replaced by a condensing boiler.

Condensing boilers explained

When a boiler is referred to as 'condensing', it is a way of describing how the boiler operates.

Boilers that have a 'Flue Gas Recovery Unit' fitted to them are condensing boilers. In simple(ish) terms, this means that exhaust gases are recycled through the boiler's condensing heat exchanging, ensuring that as little heat as possible is wasted. Compare this to old, non-condensing boilers where the exhaust gas goes out through the flue and as much as 30% of heat is wasted.

The fact that the exhaust gases are recycled makes the condensing boiler much more efficient than its non-condensing counterparts. The difference in efficiency is pretty massive with modern condensing boilers achieving 90 - 99% efficiency, whereas even the most modern of non-condensing boilers can only reach a maximum of 78% efficiency. Old non-condensers are even worse, typically achieving only a maximum of 65% efficiency.

 In short, condensing boilers will use far less gas and result in much lower energy bills than non-condensing boilers. This is one of the main reasons non-condensing boilers are no longer available for installation.

To put it in perspective:

  • If a boiler is operating at only 65% efficiency, for every £1 you spend on energy, 35p is wasted.
  • Whereas, if a boiler is operating at 99% efficiency, for every £1 you spend on energy, only 1p is wasted.

Combi boilers

A combi (combination) boiler is a type of condensing boiler.

They are all-inclusive boilers that combine everything you need to heat the home and provide hot water in one unit. Combi boilers are particularly suited to flats and small houses, where the demand for hot water and heating is naturally lower than in a large family home. 

Every combi boiler contains a heat exchanger which heats water instantaneously and provides it on-demand to hot water taps and showers at a high pressure. Simultaneously, it can take water from the mains, heat it quickly and deliver it to your radiators to heat your home. This is an incredibly efficient way of providing heat and is why they are recommended for use wherever possible.

Other types of boiler (like regular or system boilers) require an additional hot water cylinder, often located in an airing cupboard. In order for you to receive hot water when you shower of run a shower or tap, the water must be kept hot within the cylinder. This of course uses more energy than the instantaneous heat produced by a combi boiler. A water cyclinder also takes up space, which is simply not an issue with combi boilers and another advtanage of their installation.

However, combi boilers don't work so well in larger homes with multiple bathrooms or where demand for heating is higher, as they can't keep up with the amount of hot water required. This is where regular or system boilers are more suitable.

Regular boilers

If you have a very old boiler in your home, it is very likely to be a regular (aka 'standard' or 'conventional') boiler. It will probably be one of the old non-condensing boilers referred to earlier in this article.

A central heating system with a regular boiler can most commonly be identified by the fact that there is a hot water cylinder in an airing cupboard with an immersion switch and a cold water tank in the loft. Some regular boilers have timers so that you can set when you want hot water for washing, while some older ones will require you to wait while the water heats up.

While regular boilers are now considered inefficient and outdated technology, many heating engineers like them for their relative simplicity to fix and this makes them cheaper in the short term to maintain. Don't be surprised to hear comments like "this is when boilers were built to last" from your heating engineer regarding regular boilers.

System boilers

System boilers are a bit of a more modern version of regular boilers and are nowadays usually condensing boilers, equipped with the Flue Gas Recover Unit. They still require a separate hot water cylinder to accompany them but they don't require a water tank in the loft - great if you're planning a loft conversion. 

These are more efficient than regular boilers and are generally recommended for larger homes that need a new boiler installed but where a combi boiler wouldn't be suitable.

What boiler should you get?

Because of the energy efficiency and space-saving advantages, a combi boiler will always be the go-to choice. However, as explained above, a combi boiler might not be suitable if you've got a large home and multiple bathrooms that may be in use at te same time. In this case you should consider a system boiler.

You'll also need to think about the size of boiler you'll need to adequately heat your home.

The average combi boiler is available in 24-25kw, 28-30kw, 33-35kw and 40kw.

As a rough guide most apartments and smaller houses, with up to 10 radiators will require a 24-25kw boiler.

A 28-30kw would be installed for a medium to large 3-4 bed house with up to 15 radiators, and a 33-35kw and a 40kw would be for a large house with anything up to 20 radiators.

The best thing for you to do is to speak to a professional Gas Safe Registered heating engineer to establish the type and size of boiler that would best suit your property and lifestyle.

New radiators from TradeRadiators.com

If you're replacing the boiler in your home, it's likely that you'll need to upgrade the radiators at the same time. If you need help with deciding on the right radiators for your new central heating system then please give us a call on 0141 225 0430. Our helpful team will be able to advice you on dimensions, heat outputs and styles to suit your rooms.

At Trade Radiators we want everyone to find the ideal radiator for any space, whether you’re looking for a no-fuss simple heating solution to the box bedroom upstairs, or fancy updating your old bathroom panel radiator for a sleek new chrome towel rail.

With the best range of radiators and valves, you’ll find from any UK store, better delivery and our unique price promise, we aim to help any room across the country get the radiator it deserves. It all starts with knowing what you need from a new radiator.

4 October 2022