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Faulty Heat Exchanger On Boiler Causes, Costs & How To Fix

Faulty Heat Exchanger on Boiler

What are the symptoms of a damaged heat exchanger?

Welcome to our simple guide to help diagnose if you have a faulty boiler, heating system, or particularly the dreaded heat exchanger.

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If you have a faulty or leaking heat exchanger, you are probably thinking to yourself how much is this going to cost me?? The quick answer is – a lot.

If you are wondering why? let’s answer all your questions such as “what is a heat exchanger? how does it work? how should you protect it and what are the signs that it is faulty?”

Don’t panic carry on reading and you should understand as much as you need to know about a heat exchanger and whether you need to repair or replace yours and how much you can expect to pay.

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Can you repair a heat exchanger?

replace heat exchanger on gas boiler

You can get a quote and advice from a Gas Safe registered engineer. Remember to get a couple of quotes to compare the cost of replacement as they all charge differently.

If you would prefer to have a new boiler and heating system fitted you can contact compare boiler quotes today. We are the only company in the UK that can offer Viessmann boilers with a massive 15-year warranty.

Did you know a boiler service keeps your gas bills down by keeping your boiler energy efficient as well as stopping things like a fault heat exchanger from happening? Book yours today.

Fill in the form if you want a call back to book a service for £80 nationwide, or £70 if you sign up for an annual service plan for £8.99

How Much Is Boiler Heat Exchanger Replacement?

The good news here is that a heat exchanger can be replaced, however, the actual cost of replacing a heat exchanger is at least £500, especially for top brands like Worcester Bosch, Viessmann & Ideal. This is because the heat exchanger itself is one of the most expensive parts to buy.

The actual replacement by a Gas Safe registered heating engineer shouldn’t take too long, but it may be time to consider whether the cost of boiler replacement is a better option, especially if your boiler is over 10 years old.

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What Is A Heat Exchanger in a Boiler?

As we are probably all thinking the same thing lets get straight to the point…

What is a heat exchanger?

A heat exchanger in the gas boiler works by converting the heat from gas, to transfer to your taps.

Water passes through the boilers heat exchanger, heating up the temperature in the process.

The water is fed into the central heating system through a pipe know as a “flow” pipe.

It circulates around the heating system and returns via the “return” pipe.

As the water passes through radiators, pipework, and any other heating essentials it loses the hot temperature.

When needed as it enters the heat exchanger again, it’s reheated, and the process continues.

Unfortunately, a heat exchanger is not cheap to fix and it may be better to get a new boiler replacement in the long term.

Common issues with a Heat Exchanger

Boiler heat exchanger leak

A leaking heat exchanger is the first sign that it is broken. You won’t actually see the heat exchanger leaking, you will see water dripping from your boiler underneath.

This may not be your heat exchanger, but the only way to find out is to get a qualified engineer to look at it.

Like any other mechanical mechanism the heat exchanger can develop faults.

Other than the general wear and tear, there are two major things that cause heat exchanger problems.

  • Limescale build-up.
  • Heating sludge build-up.
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Problems with Limescale Build-up in heat exchanger

Do you live in an area that is considered to have moderate or hard water?

This is usually more in the countryside but make sure you check with your water supplier.

If so it is very likely that limescale has built up on the heat exchanger.

Limescale can be fed into the system through minerals in the water.

The limescale which you may have seen looks white and chalky, can then attach to taps, radiators and pipes.

Symptoms Of Limescale Build-up

As expected the first sign of an issue could be an unusual noise. Sounds similar to a kettle boiling hence the names.

  • Kettling
  • Whistling

This is a sign of limescale build-up that has created a “hotspot” on the heat exchanger.

The noise you are hearing is steam bubbles from the water being overheated.

Boiler Lockouts

Every boiler is fitted with an NTC thermistor.

The flow pipe thermistor monitors the temperature of the water that is exiting the boiler.

If this temperature is too high, it will lock out.

When a heat exchanger develops limescale build-up, the water can overheat.

The thermistors’ job is to recognise water temperature, if the temperature is abnormally high it shuts the boiler down.


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

This can cause the exchanger to leak water from the boiler.

If this is the case, you’ll notice the boiler leaking from the bottom of the casing.

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Ways To Fix And Prevent Limescale Build-Up In a Heat Exchanger

Prevention is often better than the cure, but there are ways you can stop or slow down the build-up of sludge, limescale, and other debris that could cause your boiler to break down.

Her are a few simple ways that are far more cost-effective than waiting for your heat exchanger to break or even having to buy a new boiler, which is often the most likely outcome of a broken boiler.

Can a boiler heat exchanger be cleaned?

Have your boiler serviced annually with a Gas Safe heating engineer, they will run through checks to ensure everything is working correctly and prevent any breakdowns.

Heat exchanger fault is unfortunately bad news.

Heat exchanger repair costs can be around £550 upwards.

It doesn’t often make sense to spend that much money on an old boiler and would usually be recommended to replace.

On the other hand, hopefully, you’ve caught the problem as soon as possible and an engineer could help with the following:

  • Use limescale silencer (such as Sentinel X200), this will breakdown limescale in the system as it acts like a cleaner.
  • Fit a limescale reducer mechanism, this will collect limescale as it passes through the filter.
  • Have the engineer take a look at the heat exchanger.

Central Heating Sludge

Limescale comes from the water but central heating sludge builds up within your heating system.

Sludge is built up of rust and dirt breaking off from copper pipes, joints and other particles.

The sludge will create a blockage which will then cause a fault with your heat exchanger.

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Symptoms Of Heating Sludge Build-up in the heat exchanger


Finding a sludge blockage will be similar to sourcing a limescale issue, noise.

The difference will be the sound; Tapping & Scratching

The bits of rust will be passing through rather than attaching themselves to the boiler’s heat exchanger.

Boiler Lockouts

In the same way the limescale build-up causes a lockout so does the sludge.

It’s more likely to be a “water circulation” fault rather than the water overheating.

Heating sludge will be restricting water flow.

The heated water isn’t able to travel around the system as quickly and easily as it should be due to the blockage.

When this happens it’s more than likely the temperature difference between the flow and return pipes is higher than it should be, causing the boiler to lock out.

Bleeding Radiators

When bleeding radiators air is released first and then followed by water.

If the water coming out of the radiators is black, you know this is heating sludge that is causing the blockage.

Ways To Fix And Prevent Heating Sludge Build-Up

Limescale build-up usually causes more severe boiler heat exchanger problems than heating sludge does.

Both issues need to be sorted as soon as they are noticed.

A Gas Safe heating engineer would be needed to take care of this problem.

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Heat Exchanger Repairs

Unfortunately, the word “expensive” is used here.

Both the parts and labour for this job are extremely expensive unless:

  • The boiler is new, it will be under warranty, so you won’t have to pay to fix it.
  • If the boiler is old, there’s no point in investing in a unit that is slowly deteriorating, you should replace the boiler.

How Much Does It Cost To Replace A Heat Exchanger?

Repair isn’t usually an option, you would need a replacement heat exchanger. Most people would actually opt for a new boiler as the cost of a new heat exchanger is about a third of the price of a new boiler. 

Expect to pay anywhere from £500 for a new heat exchanger for your boiler unless it is under warranty. If it is under warranty the cost should be included in any guarantee. 

Faulty heat exchangers – What’s Next?

We hope our heat exchanger guide was helpful, for further information and to find out the cost of a new boiler please click below to get fixed prices on the best combi boiler, system boiler and conventional boilers. We compare all the top gas boielr brands like Viessmann who only use stainless steel heat exchangers for reliability and longevity.

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