Home  >  Advice Centre   >   Faulty Heat Exchanger

Faulty Heat Exchanger


A boiler comprises different components that all gel up and syncs into one to efficiently provide warmth to your home.  One of these components is the heat exchanger, and if it develops faults, it spells trouble for you as they are costly to replace. In your best interest, you should identify potential problems in your heat exchanger way ahead of time before it comes out of commission.

What is a heat exchanger?

A heat exchanger converts the heat energy from the heated gas into heat to warm up the cold water. Water is fed through the flow pipe to a series of spiral pipes that the hot gases flow over for efficient heat exchange.

The now heated water circulates the boiler system, passing through the pipework, radiators, and the heated tower rails before coming back to the boiler. During this process, water loses heat while warming your home and returns to be heated again in the heat exchanger via the return pipe. This process is repeated consecutively, and through it, an optimal temperature is maintained in your home.

Common Heat Exchanger Problems

Like any other internal part of a boiler, the heat exchanger can become faulty over time. In most cases, the fault in the heat exchanger can be traced to wear and tear, an inevitable problem. However, there are some tell-tale signs you should look for and kick into action once you identify them to prevent the festering of any initial issues.

But before we get to that, let’s look at the three major causes of heat exchanger problems which include:

  • Limescale build-up.
  • Heating sludge build-up.
  • A cracked heat exchanger.

Limescale Build-up

Limescale is the main culprit of heat exchanger problems if you live in an area with hard water. The chalky white substances in hard water can wreak havoc on appliances and pipework and lead to catastrophic proportions on crucial components over time.

Therefore, if the water flowing through your boiler system is hard water and contains lots of these minerals, your heat exchanger is highly susceptible to faultiness. It is because the heat exchanger is constantly exposed to hard water and prone to limescale deposits.

Common signs of limescale build-up

Whistling and Kettling noises – Over time, the limestone deposit can cause overheating and hotspots within your heating system. The noise results from steam and water bubbling over the hotspots and creating a distinctive sound similar to a boiling kettle.

Leaky heat exchanger – The overheating caused by the limescale deposits can eventually lead to the development of cracks and broken seals in your heat exchanger or other components. These cracks and broken seals will result in leaks usually deposited at the bottom of the boiler casing. It is brought about by the unusual expansion and contraction of the heat exchanger, thereby loosening its structure.

Boiler lockouts

As earlier mentioned, limescale build-up causes overheating and hotspots in your heat exchanger and other components. Most boilers are fitted with an NTC thermistor, whose sole purpose is to monitor the temperature of the water exiting the boiler. If the thermistor detects an unusually high temperature in your system, it shuts down the boiler and locks you out.

How to fix and prevent limescale build-up in the heat exchanger

One way to solve and prevent limescale build-up issues is to have your boiler serviced annually by a Gas Safe Heating Engineer. The engineer will run checks and ensure your boiler and other components like the heat exchanger work correctly, thus avoiding an unprecedented breakdown.

Regrettably, most people discover limescale build-up too late when the damage has been done. It is catastrophic as the only course of action is to install a replacement heat exchanger. It is easier said than done as heat exchangers can be expensive, especially if a complete replacement is required.

In these circumstances, getting a new boiler altogether would be more cost-effective than patching up an old boiler. If your boiler is new, the warranty will cover your replacement and labour costs. If you suspect limescale build-up, you should seek the assistance of a qualified professional heating engineer.

The heating engineer could help with the following:

  • Use a limescale silencer that breaks down the limescale in the heat exchanger and the central heating. It essentially works like a cleaner and should clear up small limescale build-up.
  • Fit a limescale reducer mechanism. It works like the magnetic filter and collects up limescale floating around the heat exchanger and the central heating and prevents a build-up.
  • Look at the heat exchanger itself. The heating engineer may clean the component, but if the build-up is too severe or the heat exchanger has already cracked, you will need to get a new one.

Central Heating Sludge

Central heating sludge is a prevalent problem, especially with older boilers. It happens when dirt and rusty metal break away from the copper pipes and build up within the system resulting in a thick brown substance known as sludge.

Sludge may impede hot water circulation in the system resulting in cold spots, or worse, over time, the sludge may cause corrosion to your heating systems and result in blockages. The heat exchanger is especially susceptible since the heating water is constantly passed through them. If the water contains sludge, it leads to sludge build-up in the heat exchanger that cannot be averted.

Common signs of heating sludge build-up


If you have sludge build-up within your system, noise is likely made in the boiler or elsewhere. The noise produced is characteristically different from that produced from limescale build-up. If the noise comes from the heat exchanger, it is likely caused by pieces of sludge passing through the heat exchanger and scrapping against it.

As a result, it sounds more like scratching and tapping noises rather than whistling or bubbling.

Bleeding dirty radiator water

Bleeding your radiators is the best way of gauging the quality of water circulating within the system. It also affirms the absence or presence of sludge in your heat exchanger and system. When bleeding the radiators, the air is released first and then water. If the water coming out is black, it is a clear indication of sludge build-up that may be causing the blockage.

Boiler lockouts

A boiler lockout is essentially a failsafe mechanism that sets in if the boiler senses a fault within its system. A build-up of sludge in the heat exchanger will result in boiler lockout. In some cases, the trigger can be caused by an overheating issue, but it is more likely to be caused by a water circulation problem. In this case, the debris in the water impedes the effortless flow of water through the heat exchanger and other components.

You can check this by taking note of the temperature difference between the flow and the return pipe. If water flow is being blocked, the return pipe will likely feel a lot colder than usual.

How to fix and prevent heating sludge build-up

Heating sludge can significantly reduce your boiler’s efficiency and performance; however, it is a relief that it is not as destructive and detrimental as limescale build-up. That said, it still needs to be sorted out to avoid deep-running problems in the future. You will need to get a qualified heating engineer to help you out.

The qualified heating engineer could help with the following:

  • Inspect the heat exchanger and clear any sludge and debris that build-up there
  • The engineer can run a hot water flush or a power flush to clear out sludge from other system parts. However, beware of running this procedure on older boilers as it’s too powerful and can damage older central heating parts.
  • Dose your system with an inhibitor. Once the system has been flushed, the inhibitor helps clean things out and prevent sludge build-up in the future. It is vital in averting a similar occurrence.
  • Fit a magnetic system filter. It collects bits of metal debris floating around in your central heating and helps prevent future sludge build-up. The filter will need to be cleaned sporadically and done as part of your annual boiler service.

A Cracked Heat Exchanger

Cracks in the heat exchange are usually caused by normal wear and tear. The heat exchanger experiences intermittent expansions and contractions that loosen its structures, leading to cracks. A cracked heat exchanger poses a severe threat to your home as dangerous gases such as carbon monoxide, sulfur dioxide, and nitrous oxide could leak into your home with devastating effects.

Signs your heat exchanger may be cracked.

  • Strange smells – A malfunctioning heat exchanger (a cracked one) will often produce an unpleasant and pungent odour that smells similar to formaldehyde.
  • Soot build-up – White soot build-up in and around the burner indicates a crack in your heat exchanger.
  • Corrosion and cracks – Sometimes, gaps are evident on the heat exchanger or other components.
  • Change in the flame appearance – You can see flames that heat your air in most central heating. A blue flame is normal; however, if you notice an orange or yellow flame is an indication of a crack.
  • Audible sounds- You are likely to hear a rattling noise if you have a crack in your heat exchanger as you turn the thermostat. Expansion of the heat exchanger causes noise.
  • Leaked gases- You should install a carbon monoxide detector or alarm that will alert you if your heat exchanger leaks of any dangerous gases.

If you detect any of the signs above, you should immediately contact a qualified heat engineer as cracks in the heat exchanger are potentially dangerous.

Heat exchange repairs

Unfortunately, repairing heat exchangers for any of the problems mentioned above is an expensive affair. The parts and the labour for this job will dig deep into your pocket. If the boiler is new, you will be covered by the warranty. However, if it is old, shelling out a replacement for the heat exchanger will cost around £400. Buying a new one is the best option rather than patching up an old boiler which is likely to break down soon.

  • Buy Now Pay Later Schemes!
    • Interest Free Credit
    • Buy Now Pay Later
    • Spread Payments
    Finance Options
  • Free Boiler Quotes

    No-obligation to buy!

    • From the comfort of your home
    • You choose the best company
    • Gas Safe trusted companies
    Get Quote NOW
Need a New Boiler

Get FREE quotes from tradesmen in your area today. What are you looking for?