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Faulty boiler heat exchanger: Here's what they are (with costs to fix)

Think your boiler's heat exchanger is busted? Here, we'll exchange some advice on what to look for, the likely causes and the (gut-wrenching) costs to fix, or replace it.

The heat exchanger is an essential component in boilers, responsible for allowing the efficient transfer of heat from burning gas to cold water. Without it, your boiler would fail to heat your water - which, is kind of important.

Unfortunately, when the heat exchanger is faulty, a boiler can fail at its job (like Donald Trump during a pandemic), leading to cold or lukewarm water. 

So why do heat exchangers fail? When do you know it's failed? Whom can fix it and what cost?

So many questions.

In this article, we'll do our absolute bestest to answer them.

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First thing's first, what is a boiler heat exchanger? 

What is a boiler heat exchanger?

As the name implies, a heat exchanger transfers heat between two substances, while preventing them from making physical contact. In the case of gas combi boilers, this is between hell-like flames and cold water. 

Inside of the heat exchanger is a coiled pipe, which is surrounded by the cold water, ready to be heated. Once the hell-like heat from the flames passes through, the water heats up. This system keeps the substances separate, which is super-efficient.


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Heat exchangers are found in a wide variety of different appliances including boilers, swimming pools and even air conditioning units. In the case of air conditioning, the process is simply reversed, in order to cool instead of heat - clever huh?

How does a heat exchanger work? 

How does a boiler heat exchanger work?

As the gas burns, the heat rises to meet the heat exchanger. This coiled structure has cold water flowing adjacent to it, so as the water circulates the heat is exchanged from the burning gas to the cold water. In turn, this produces the hot water supply for both hot water and heating.

How a Heat Exchanger Works

This water is then fed into the heating system via a flow pipe and as it circulates through the system e.g. the pipes and radiators, it loses its heat and is returned to the boiler to be reheated again.

So, in simple terms, the process begins with hot gas and cold water, and thanks to the heat exchanger we end up with cooler gas and hot water without the two substances ever meeting. 


Common heat exchanger faults   

Common heat exchanger faults

Just like any other internal boiler components, over time, wear and tear can take its toll and result in damage and disrepair. 

The heat exchanger is no different, yet unlike other components, the majority of heat exchanger damage results as a consequence of poor-quality heating water. 

If your current boiler has failed, Heatable install new ones from £1,545 - get your fixed price online.

Limescale build-up 

Limescale is not usually an issue unless you live in an area with hard water, you can check using this map

Hard water areasThis chalky white substance can wreak havoc on appliances and pipework, over time building up and leaving deposits on key components. 

The water flowing through your heating system and boiler can be an issue if it contains lots of these minerals. It is particularly troublesome for heat exchangers, as they are constantly exposed to the hard water and prone to limescale deposits. 

Signs of limescale build-up 

So how can you detect whether limescale build-up is an issue in your boiler? 

If you live in an area with limescale and have not routinely cleaned your boiler, chances are it is going to be an issue at some point. 

With that said, here are some of the most common signs of limescale build-up: 

  • Whistling and kettling noises 
    When limescale deposits build up, they become overheated, which can result in the production of steam and bubbling, causing a familiar whistling noise, referred to as kettling
  • Leaking heat exchanger 
    The excessive heat can also result in the development of cracks and broken seals in the heat exchanger or nearby components, resulting in a leak. These types of leaks will typically deposit at the bottom of the boiler casing, often needing to be emptied periodically. 

How to deal with limescale build-up  

Unfortunately, many people only discover they have limescale build-up when the damage has been done and there is no other option but to install a replacement heat exchanger. 

Heat exchanger repairs can be expensive, especially if a complete replacement is required. In these circumstances, it often makes sense to get a new boiler rather than spend so much on an old boiler, especially if its warranty has already expired. 


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If you suspect limescale build-up, you can consult the assistance of a Gas Safe Engineer who can: 

  • Clean your system with a limescale remover, from a brand such as ADEY
  • Carry out a thorough inspection of your boiler (including the heat exchanger) to check for damage
  • Installation of a limescale inhibitor

Is your boiler making a lot of noise? Find out why in our noisy boiler guide. 

Central heating sludge

Over time, rust and dirt can build up within your heating system, resulting in a thick, brown substance called sludge. 

Sludge can slow down the flow of water within your heating system, resulting in annoying radiator cold spots. Even worse, over time the sludge can cause further damage to your system, resulting in corrosion and blockages.

Heat exchangers are especially susceptible since the heating water is passed by them constantly, so any sludge contained is going to make contact. 

A power flush, or system cleanse can fix sludge.

Signs of sludge in your system 

What are some of the major signs of sludge in your system? 

  • Kettling
    If you have noticed a kettling noise coming from your boiler e.g. like a kettle boiling, then it is possible that sludge could be the culprit. As the sludge passes through your boiler, it can become deposited on the exchanger, gradually building up until it eventually starts to restrict the flow of water. 
  • Dirty radiator water
    When bleeding your radiators, you can usually gauge the quality of your heating water quality quite well. If the water you bleed is viscous and dark in colour, it is highly likely your system has a sludge problem. 

Learn all about combi boilers in our what is a combi boiler guide. 

How to deal with central heating sludge 

If your system’s heating water quality is extremely low and contaminated with the likes of sludge, rust, and other harmful residues, it will benefit from a deep clean. 

It is advised that your heating system receives a “flush” every 5 years, but if you suspect an issue it is a good idea to have a Gas Safe Engineer carry out an inspection sooner. 

An engineer can take the following measures to help investigate and deal with a potential sludge build-up: 

  • Inspection and cleaning of the heat exchanger
  • A complete deep clean of your heating water to improve quality and remove harmful sludge, for example, a power flush
  • Installation of a filter which will continually collect any harmful residues e.g. the ADEY MagnaClean Pro 2

Can you repair a broken heat exchanger?

The reality is that it typically does not make much sense to repair a boiler with a broken heat exchanger as the costs of the parts and labour are expensive. Here is the thing: 

  • If your boiler type is relatively new it is likely to be under warranty and so any repairs will be covered. 
  • If the boiler is old, it will have no warranty and is likely to be in continual decline – why keep throwing money at something that is in free fall? 

How much does it cost to replace a boiler heat exchanger? 

Typically, new heat exchangers can range anywhere between £400 - £600, so it often makes more sense to buy a brand-new boiler, especially if your current one is an older inefficient one with an expired warranty. 

Curious to know which boiler brand performed worst? Find out in our report of the worst boilers

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