Faulty Boiler Heat Exchanger – How to Diagnose And Fix

Problems with the heat exchanger are one of the most common faults on most boilers, but a lot of people don’t even know what it does, much less how to fix it. In this guide, we will tell you everything you need to know about heat exchanger, what faults they can develop, and how to get them fixed.

If you have never heard of a heat exchanger, you are not alone. Most people are unfamiliar with the term, don’t know what a heat exchanger does, or how it can go wrong.

But actually, a faulty heat exchanger is one of the most common faults on any boiler, regardless of which make and model you have.

In this guide, we will explain what your boilers heat exchanger does, what can cause it to go wrong, and outline the steps you will need to take to fix it.


What Is A Heat Exchanger?

The heat exchanger is the component within your boiler that converts the energy the boiler creates from its gas supply into hot water.

Water passes through your boiler’s heat exchanger and when it does so it heats up, rather like it does in a kettle. This water is then fed into the central heating system through your boiler’s flow pipe. It circulates around the heating system, passing through all your radiators and towel rails before returning to the boiler via the return pipe.

It then passes through the heat exchanger again and the process is repeated.

It goes without saying that this is a critical part of your boiler’s work and if the heat exchanger is not functioning properly, your boiler will not be able to provide central heating and hot water to your home.

Common Heat Exchanger Faults

The heat exchanger is one of the most active components in your boiler and it is therefore perhaps inevitable that it is one of the parts that goes wrong most frequently.

There are a variety of faults that can develop but there are two major issues that tend to crop up most regularly:

  • Heating sludge build-up.
  • Limescale build-up.

Central Heating Sludge

Central heating sludge is a common issue with most heating systems but tends to be most common in older ones.

It happens when of dirt and rusty metal break away from your pipes, radiators, or joints, and end up in the water going around your system.

Over time, this can develop into a sludge buildup in various places around your system, including in the boiler itself. If the build-up is big enough it can cause a blockage or stop components in the boiler from working properly.

Symptoms Of A Heating Sludge Build-up


If you have a build-up of sludge in your system, it is likely to result in a noise being made either in the boiler or elsewhere.

If this noise is coming from the heat exchanger, it is most likely to be caused by pieces of sludge or debris passing through the heat exchanger and scraping against it. As a result, they usually sound more like scratching or tapping noises than anything else.


A boiler lockout is a failsafe mechanism that will kick in if the boiler senses that there is a fault. A build-up of central heating sludge in the heat exchanger can result in a boiler lockout.

The trigger can sometimes be because the water is overheating, but it is far more likely to be a water circulation problem. In other words, the sludge and debris is restricting the water flow through the heat exchanger and around your system.

If this is the problem you have, you can often tell by checking the temperature difference between the flow and the return pipe. Usually, the return pipe is a little colder than the flow pipe but still warm.

However, if there is a blockage that is slowing the circulation of water, the return pipe is likely to feel much colder than usual.

Another good way to check is to bleed your radiators. Once the air has been released, some water will come out. If this water is black, you can be pretty certain that there is a sludge build-up somewhere in your system.

How To Fix It:

Heating sludge can badly affect the performance of your boiler but it isn’t usually as serious a problem as limescale.

It is something you need to get sorted though and you will need to get a qualified heating engineer out to do the work for you.

They will be able to inspect the heat exchanger and clear any debris and sludge that has built up there. They are also likely to run a hot flush or a power flush which will clear any debris from the rest of your system too.

Beware though, a power flush is very powerful and can damage older central heating systems, so be cautious about running one of these.

Once the flush is complete, they should then dose your central heating system with inhibitor which helps to clean things out and prevent sludge build-ups in future.

Lastly, ask your engineer to fit a magnetic system filter. This collects up bits of metal debris floating around in your central heating systems and helps to prevent future sludge build-ups.

This filter will need to be cleaned intermittently, but this is something that should be done as part of your annual boiler service.

Symptoms of a Limescale Build-up

Limescale is something that can build-up in any central heating system but it is far more common in hard water areas than anywhere else. If you aren’t sure if you live in a hard water area, check out this map to see.

If your area is marked as a hard water are or even just moderate, it is quite possible that limescale is causing your heat exchanger problems.

Limescale comes from minerals which are found in all water, but which is most common in hard water. This water flows around your system and these minerals can attach themselves to the insides of your radiators, towel rails, pipes, and boiler components including the heat exchanger.

Over time, this build-up forms a substance known as limescale and it can cause components to break down and stop working properly.


Like a sludge build-up, limescale on your heat exchanger is likely to cause it to make a noise. But this noise will sound a little different, with a kittling or whistling noise being far more common.

This noise occurs because the limescale can cause a hotspot on the heat exchanger. This is where the water is heating up too much and the noise you can hear is bubbling being created by the water boiling inside the heat exchanger.


As with a sludge build-up, limescale can result in your boiler going into lockdown.

The most common cause of this is the NTC thermistor. This is the component that monitors the temperature of the water exiting the boiler. If limescale has created a hotspot and the water coming out of your boiler is too hot, the NTC thermistor will recognise this and the boiler will lockout.


Hotspots caused by limescale can also result in leaks. This is usually because the increased heat caused by hotspots can eventually lead to cracks occurring in the heat exchanger itself.

Water can then seep out of these cracks and you will usually spot it coming out of the bottom of your boiler casing.

The hotspots created by limescale can eventually lead to cracks on a heat exchanger.

This can cause the exchanger to leak. If this is the case, you’ll notice the boiler leaking from the bottom of the casing.

How To Fix It:

Unfortunately, a limescale build-up is another problem that will need the help of a qualified professional heating engineer.

Limescale can be a much more serious problem than a sludge buildup, so hopefully, you have caught the matter early.

If you have, your engineer will be able to use limescale silencer like Sentinel X200 to breakdown the limescale in your central heating system. This essentially works like a cleaner and should clear up small limescale buildups.

He may also fit a limescale reducer. This works much like the magnetic filter and collects up limescale that is floating around your central heating system to stop it creating a buildup.

If it is a more serious buildup, your engineer will look at the heat exchanger itself. If possible, they may clean the component, but if the buildup is too serious or the heat exchanger has already cracked, you will need to shell out for a new one.

The bad news is that heat exchangers don’t come cheap. The cost of a replacement can be easily in excess of £500.

If your boiler is still under warranty, this cost should be covered and you might just have to pay a small amount in labour costs depending on the terms of your warranty.

But if it isn’t this is a cost you will have to cover yourself.

If your boiler is quite old already, you might, therefore, want to think about whether it is worth spending this on a replacement part of whether you would be better off investing in a whole new boiler.

Replace v Renew

If your heat exchanger is beyond repair, you will be left with the stark choice of replacing the component, at a cost of at least £500 or buying a new boiler.

While a new boiler will cost more, the difference is actually not as much as you might think and in the long run a new boiler is likely to save you money.

Older boilers will go wrong more and more, and once you have replaced one major component it is quite likely that another will quickly follow.

By choosing to invest in a brand new boiler, you are signing up for a new warranty that means any repairs are covered for the duration of that deal.

New boilers are also far more energy efficient which means you are likely to see a reduction in your gas bill too.

If you are considering a new boiler, get plenty of quotes. We particularly recommend that you take a look at WARMZILLA.

WARMZILLA is an online installation company that offers some of the lowest prices on a wide range of high-quality boilers from all the top boiler manufacturers. They have some great deals available including extended warranties.

Just visit their website and fill in their short form to see what best deals are right for you. You don’t need to share any personal information and all their quotes are fixed price. They can even get your new boiler installed as quickly as the following day.


Heat exchangers are vitally important, but they can go wrong. When they do, it is likely that a buildup of sludge or limescale is the cause.

In this guide, we have explained what causes this and how you can fix it. We have also explained when the problem is too serious you might want to consider investing in a new boiler instead. If you do, we have recommended getting a quote from WARMZILLA to see how much money you can save.

Have you had problems with your heat exchanger? What was the cause of the problem and how did you fix it? Do you have any tips for our readers we haven’t covered off in this guide? Do you have any questions we haven’t answered in this guide?

It is always interesting to hear about the thoughts and experiences of all our readers, so why not share your thoughts with us today using the comment box below? We try to respond to every comment we get.

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