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Faulty Boiler Heat Exchanger

Faulty Boiler Heat Exchanger

Faulty Heat Exchanger or Boiler Faulty Boiler Heat Exchanger Compare Boiler Quotes


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 heat exchanger.

You are probably thinking to yourself “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.

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 different.

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 who can offer Viessmann boilers with a massive 15 year warranty.


What Is A Heat Exchanger? Faulty Boiler Heat Exchanger Compare Boiler Quotes

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.

Common issues with a Heat Exchanger

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.

Problems with Limescale Build-up Faulty Boiler Heat Exchanger Compare Boiler Quotes

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 a NTC thermistor.

The flow pipe thermistor monitors the temperature of water which 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 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.

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


Ways To Fix And Prevent Limescale Build-Up


Can a boiler heat exchanger be cleaned? Faulty Boiler Heat Exchanger Compare Boiler Quotes

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 Faulty Boiler Heat Exchanger Compare Boiler Quotes

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 fault with your heat exchanger.

Symptoms Of Heating Sludge Build-up


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

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 Faulty Boiler Heat Exchanger Compare Boiler Quotes

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 which is causing the blockage.

Ways To Fix And Prevent Heating Sludge Build-Up

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

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

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


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 investing in a unit that is slowly deteriorating, you should replace the boiler.

How Much Does It Costs To Replace A Heat Exchanger? Faulty Boiler Heat Exchanger Compare Boiler Quotes

Repair isn’t usually an option, you would need a replacement.

Expect to pay anywhere up to £600 for a new heat exchanger for your boiler unless it is under warranty.


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.

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