Boiler flue expelling plume in condensing mode

What Is A Condensing Boiler, And How Do They Work?


You have probably heard the term “condensing” describe boilers and wondered what this important-sounding phrase means. In this guide, we break down what condensing boilers are, how they work, and what you need to know when planning to get one installed.

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What is a condensing boiler?

Put simply; a condensing boiler can recycle the heat energy present within the combustion gasses to increase the overall efficiency of the boiler.

During operation, boilers produce combustion gasses that escape up the flue to the outside of the property.

There is heat energy in the combustion gases, and in older boilers, this heat is simply lost to the atmosphere.

However, in a condensing boiler, this heat within the combustion gas is harnessed and used to provide heat back into the boiler.

This heat recovery process means that your boiler uses less gas to produce heat and leads to energy savings due to the increased efficiency.

How do condensing boilers work?

An illustration showing how a condensing boiler works

Let’s dive a little bit deeper into how condensing boilers work.

We mentioned earlier that combustion gases are produced while your boiler produces heating and hot water. These combustion gasses are hot and usually are terminated outside your property safely via the flue.

In a condensing boiler, when these waste gases are on their way to the flue, they pass over a specially designed heat exchanger and provide additional heat to it.

This unique heat exchanger is then used to transfer heat to the cool water, returning to the boiler from the heating system circuit.

This means that any remaining heat energy from the combustion process is captured and used to aid the boiler in producing hot water for the heating system.

The effect of this process in modern condensing boilers reduces the amount of gas used and, in turn, reduces your energy bills in comparison to non-condensing boilers, which cannot harness the latent heat.

More detail about the heat exchanger

A primary heat exchanger for a gas boiler

The heat exchanger referred to in the condensing process is the primary heat exchanger within the boiler. But there can be multiple heat exchangers within the boiler.

The primary heat exchanger is there to heat water to be used within the central heating circuit. In other words, it heats the water which is pumped into your radiators.

There is also a secondary heat exchanger in combi boilers whose job is to heat the water pumped to your taps and showers.

Why two heat exchangers?

There is a necessity to keep separate the water for heating and water used in your hot water outlets. You don’t want to be showering or washing your hands with the same water circulating through your central heating system!

Are all new boilers condensing?

A gavel with the text Condensing is law!

Yes. Since the 1st April 2005, it has been the law that all new gas boilers installed in the UK must be energy-efficient condensing boilers.

The same rule came into effect for oil boilers on 1st April 2007.

This means that regardless of what boiler you choose, it will be condensing.

This is a good thing because you don’t have to worry about getting a condensing or non-condensing boiler.

There is a lot of talk among the industry about whose boiler is the most efficient, but in reality, due to the legislation mentioned above, all boilers are now high efficiency condensing boilers, and the differences in energy efficiency between different boiler types are marginal.

What is the difference between combi boilers and condensing boilers?

First of all, you need to understand that a “combi” combination boiler is a type of boiler, and “condensing” is the energy-efficient technology present within all modern boilers.

With that said, you cannot compare a combi boiler with a condensing boiler because they are the same and all other boiler types, such as system boilers.

Some confusion begins when people, typically gas engineers, refer to ‘standard boilers.’

Standard boilers

A standard boiler could have more than one meaning, as some engineers mean “standard-efficiency” when they make this reference, and some mean “heat only” (regular) boilers.

There is a distinction to be made here, as a standard efficiency boiler is simply any boiler that is not condensing. For example, this could refer to a standard efficiency combi boiler.

“Standard boiler” could also mean “heat only” (regular) boiler, which is simply a type of boiler, which has nothing to do with condensing technology, and these boilers will be condensing by law if installed after 1st April 2007.

Conventional boilers

Also referred to as a “traditional boiler,” this term confuses due to having multiple possible meanings.

When a gas engineer refers to a conventional boiler, he may be referring to a standard efficiency boiler or simply a heat-only (regular) boiler.

A standard-efficiency boiler will not be condensing, but a heat-only (regular) boiler might be condensing. If it was installed after 1st April 2005, it certainly should be, as should all gas-fired boilers installed after that date.

Suppose you are discussing a new boiler and get confused about the use of the term conventional boiler. In that case, it is worth asking whether your engineer is referring to the type of boiler or the energy efficiency of the boiler.

Bear in mind it could be both!

Rest assured that your new boiler will need to be a condensing boiler by law in any case.

Advantages of a condensing combi boiler

In addition to lower greenhouse gas emissions due to the increased efficiency achieved via condensing technology, condensing combi boilers also have the following advantages:

  • No need for a separate hot water cylinder or header tank
  • Instant “on-demand” hot water
  • Potential for even higher energy savings compared to a condensing heat-only (regular) boiler.

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How do I know if my boiler is condensing?

It’s pretty easy to check if you have a condensing boiler. If you have a plastic pipe terminating from your boiler into a drain or waste pipe, you have a condensing boiler.

Usually, it is pretty easy to tell because, in most cases, it will be the only plastic pipe terminating from underneath your boiler.

The condensate pipe may terminate outside into a drain or waste pipe, or it could terminate inside your home, into a waste pipe under your kitchen sink, for example.

Does a condensing boiler need a drain?

You might not be surprised to hear that a by-product of the condensing process is condensate production!

It is a regulatory requirement that this acidic water is discharged appropriately.

Your installer will be able to find the best route for the condensate, but it’s worth bearing in mind that it will need to go somewhere, and it can sometimes be a bit tricky to find a solution that is both compliant and aesthetically pleasing.

Internal condensate termination

The best way to terminate condensate from a condensing boiler is to connect the condensate pipe to a waste pipe inside the property.

This way, there is no need to drill additional holes in walls, and there is a much lower risk of the condensate pipe freezing.

External condensate termination

If it is not possible to terminate the condensate internally, it will need to go outside into an appropriate drain or waste pipe.

Fortunately, plastic waste pipes are now available in different colours, and so it’s a bit easier to match the colour of the condensate pipe to the property.

Risk of freezing

An illustration showing a frozen condensate pipe being thawed by pouring hot water onto it

There is always a risk of any water pipe freezing when exposed to extreme cold. Your condensate pipe is no different.

It can be particularly unpleasant to find out your boiler has stopped working due to a frozen condensate, as it is on these cold days when a hot bath or shower is most welcome!

The risk of freezing can be minimised by ensuring that the condensate pipe is lagged, and if appropriate, a heat trace wire could be installed.

Condensate summary

The trade-off for the increased energy efficiency of condensing boilers is undoubtedly the potential issue in locating a suitable method of termination for the condensate.

However, it’s a problem all new boiler installations face, and installers are well-practised in finding the least obtrusive solutions whilst remaining compliant.

Are condensing boilers worth it?

After reading our guide to condensing boilers, you may be trying to sum up whether or not you should go ahead and get one. We’ll do our best to give you our opinion on whether it’s worthwhile.

Energy efficiency legislation won’t ease off.

The government in the UK and the devolved nations have been wanting homeowners to replace their non-condensing boilers with an energy-efficient condensing boiler for some time, and there is no end in sight.

So if you are holding off with the installation of a condensing boiler in the hope that legislation will change, we think this might be a mistake.

You will save money with a condensing boiler.

Condensing boilers use less energy; therefore, the chances are that your energy bills will be lower as a result. This is the case whether you go for a combi boiler, heat only (regular) boiler, or a system boiler.

Oil-fired boilers are included too.

Condensing Worcester Bosch Oil Boiler, installed outside

It’s not just gas boilers that benefit from condensing technology. Gas or oil, you can take advantage of the benefits that come with a condensing boiler.

Condensing boiler summary

In our opinion, it is definitely worth getting a condensing boiler installed, and this is just as well because it’s the only choice by law!

You can get a quote for your new condensing boiler in under a minute.

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