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What is a heat exchanger?

installer fixing the underneath of a boiler

One of the most common places you’ll find a heat exchanger is within your boiler. It’s the crucial component which produces hot water and heating for your property. Essentially, if you’ve found yourself with a faulty heat exchanger, you’ll soon find yourself without hot water or heating.

For that reason, it’s important to take good care of a heat exchanger to ensure your boiler is running at peak efficiency, saving you money on costly repairs. Unfortunately, not many homeowners know what a boiler heat exchanger is or how to keep it maintained.

We’ve compiled a helpful guide on understanding and maintaining boiler heat exchangers, as well as spotting issues that you’ll need to act on to keep your boiler functioning at its best.

What is a boiler heat exchanger?

In simple terms, a heat exchanger allows energy (such as heat) to pass from one liquid or gas to another without the two fluids ever having to touch. This is a much more effective heating method than simply combining hot and cold water together.

A boiler heat exchanger, in essence, can be thought of as a long piece of pipe which is coiled up. The water in your heating system is pumped through this coiled pipe, hot flue gasses from the burner pass over the surface of the pipe and heat the water as it passes through.

installer showing a woman a boiler in kitchen

What does a heat exchanger do on a combi boiler?

A combi boiler (or combination boiler) provides both central heating and hot water within the same system. Without a working heat exchanger in your combi boiler, you won’t receive any hot water or heating.

Usually, a combi boiler will have two heat exchangers - one for central heating and one for hot water.

  • Primary heat exchanger. The primary heat exchanger is used to heat the radiator water, which travels to the radiator and then back to the boiler. It is consistently warmed as it cycles through the heating system.
  • Secondary plate heat exchanger. This is heated by the primary heat exchanger. When a hot water tap is turned on, water is diverted from the Primary Heat Exchanger into the secondary heat exchanger until the hot water tap is turned off.

Types of heat exchanger

As you can imagine, heat exchangers must sustain a high amount of stress when intense heat is consistently applied to the material. The most common materials you will find are stainless steel and aluminium:


Aluminium heat exchangers are lightweight and have a high thermal conductivity. The material itself is highly flexible and can be cheaper than stainless steel to install. Aluminium heat exchangers are often smaller than stainless steel types. The Aluminium is designed to be resistant to corrosion over the design life of the product

Stainless steel

Stainless steel heat exchangers are desired for their durability and very high resistance to corrosion, meaning they often maintain a longer lifespan than their aluminium counterparts. The stainless steel includes a high amount of chromium in order to be highly corrosion resistant which can impact the durability of the material.

Condensing vs non-condensing boilers

There are two types of boiler unit which function slightly differently due to the number of heat exchangers it includes.

Non-condensing boilers

Non-condensing boilers only have one heat exchanger and have a much higher carbon footprint than condensing boilers. When non-condensing boilers burn gas to create heat, the waste gases are disposed of through the exit flue, rather than recycled.

Condensing boilers

Condensing boilers have a much more efficient heat exchanger design, it still uses the same basic principle of the coiled pipe, but it can extract much more energy from the flue gasses passing over it. Put very simply it’s a longer coil of pipe which means more energy can be transferred to the water passing through it. This means that very little energy is lost through the flue.

It is now a requirement of the UK Government that all newly-fitted boilers must be a condensing unit, though exemptions may be granted.

Faulty boiler heat exchanger symptoms

Whether the material is aluminium, stainless steel or otherwise, we recommend that your heat exchanger is checked every 12 months by a gas safe engineer to identify issues before they reach urgency. Without a professional review, it can be difficult to know the ‘health’ of your heat exchanger, though there are some clear signs we recommend looking out for.

  • Noises coming from boiler

    If you have started to notice strange noises coming from your boiler, this may be indicative of a faulty heat exchanger. Listen out for high-pitched whistling noises, also known as ‘kettling’. This happens due to limescale build-up and is a clear sign your exchanger needs to be cleaned. Tapping noises may be a sign of central heating sludge, which causes pieces of rust and debris to pass through the system and cause a blockage.
  • Leakages

    Noises from the boiler can be a sign of a build-up of debris in the heat exchanger. Without being serviced, this could eventually lead to your heat exchanger leaking. If your boiler is quite old and water is constantly leaking from the bottom of its casing it could be a sign that the heat exchanger is faulty, meaning a replacement may be required. On modern boilers it is sometimes harder to identify a cracked or leaking heat exchanger. This is best left to a qualified professional to identify any issue.
  • Dirty radiator water

    Dirty radiator water can cause a blockage and, unfortunately, serious damage to your boiler. You’ll be able to see this by bleeding your radiators, where the water will appear black or discoloured. This is caused by central heating sludge, a collection of rust, debris and residue from copper pipes and radiators.

How to clean your heat exchanger

Keeping a clean and maintained exchanger will prolong the life of your boiler. This rings true for whichever boiler or type of exchanger you have, as there is always the risk of dirt or debris from your radiators.

You’ll need to get in touch with a Gas Safe registered engineer to clean the device for you. Never remove a heat exchanger yourself, as it is not safe to do so and you could cause unnecessary costly damage to your boiler.


Whilst we strongly recommend you do not remove or clean the device yourself, you can take steps to reduce the risk of limescale build up. Talk to your heating engineer about installing a limescale reducer to act as a filter or use a limescale silencer to remove build-up if the problem is further advanced.

Central heating sludge

Central heating sludge causes rust and debris to infiltrate the water. It is recommended that your heating system receives a flush every 5 years to reduce build-up of contamination and damage to your heat exchanger. A filter can also be installed to help catch debris as it passes through a filter.


  • A boiler heat exchanger is the essential component that produces hot water and heating within your boiler
  • Signs of a faulty exchanger include strange noises, leakages and dirty radiator water
  • To increase the lifespan of your boiler, ensure your heat exchanger receives annual maintenance checks from a gas safe engineer

If you notice your heat exchanger is faulty, contact your local heating engineer