Faulty Boiler Heat Exchanger: Problems And Cures

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Last updated: October 31, 2023

Your boiler is comprised of many individual parts, all of which are important because they work together to allow the boiler to heat your home and provide hot water for your taps. And one of the most important components in any boiler? The heat exchanger.

Unfortunately, problems with the heat exchanger are quite common, even amongst the expensive boiler brands and models. However, if you suspect that your boiler heat exchanger might be broken or damaged, there’s no reason to panic because in this guide, we cover everything you need to know on how to properly diagnose and fix a faulty boiler heat exchanger. We also talk about the heat exchanger repair costs, and how long you can expect your new component to last if you replace it.

What is a Boiler Heat Exchanger and How Does it Work?

Before we jump to the most common symptoms and causes of a faulty heat exchanger, it’s important to understand exactly what a heat exchanger is and how it works.

As the name suggests, the heat exchanger allows the heat to be exchanged between water and gas; or to be more precise, it transfers the heat from burning gas to water. This heated water is then fed into your central heating system via a flow pipe, where it moves through your pipework, radiators and towel rails, losing temperature as it circulates. Eventually, the water reenters the heat exchanger (via a return pipe), which heats it again, continuing the process.

As you can see, the heat exchanger is subject to frequent temperature rises and drops, which are causing the metal to expand (when hot gas is flowing through it), and contract (when the water begins to cool down). Constant expansions and contractions can cause the metal to weaken over time and even to crack. This is particularly true for heat exchangers that are not built from quality stainless steel. Unfortunately, it can be difficult to determine whether your boiler heat exchanger is faulty since you can’t see it. But thankfully, there are some symptoms that you can watch out for before calling a heating engineer.

Symptoms Of a Damaged or Faulty Boiler Heat Exchanger Include:

  • Leaking water: a cracked heat exchanger will leak water, which will eventually find its way out of the boiler casing. If you notice water on, around, or below your boiler, call a Gas-Safe registered heating engineer immediately.
  • Strange smells: a damaged heat exchanger can sometimes release strange and unpleasant odours, which can cause headaches and other physical symptoms such as watery eyes.  If you notice any strange smells similar to that of formaldehyde, contact a boiler engineer.
  • CO2 alarm is sounding: a cracked heat exchanger can also release carbon monoxide (CO2). If a CO2 alarm near your boiler is sounding, turn off your boiler and call an engineer. Do not turn the boiler on until the unit has been checked by a professional.
  • Odd noises: if you have hard water, limescale buildup could be the reason why your heat exchanger is not working properly (more on that below). If you can hear whistling or banging noises, there’s a good chance limescale buildup is to blame.

If you suspect that your boiler heat exchanger might be faulty, do not try to open the casing – this is a job for a Gas Safe registered heating engineer. Besides, if you open the casing, you could void the warranty.

Common Boiler Heat Exchanger Problems

Now that you have a clearer picture of what a faulty boiler heat exchanger can look like, let’s talk about the common reasons behind heat exchanger problems. Aside from general wear and tear, there are two main problems that can cause boiler heat exchanger faults.

Limescale Build-Up

As mentioned, if you live in a hard-water area, there’s a good chance that limescale buildup is to blame for your boiler’s heat exchanger problems. This chalky deposit forms from minerals in the water and can attach itself pretty much anywhere, from your boiler and radiators to your taps and kettles. A build-up of limescale can reduce your boiler’s efficiency, costing you more to run it. To avoid costly repairs and expensive bills, pay attention to the following symptoms.

Signs of limescale build-up

  • Kettling or whistling noises: limescale can cause blockages and inefficiency, which, among other things, can cause the water to get hotter than normal. As it comes to a boil, the build-up of steam and bubbles will cause a whistling or kettling noise. If you can hear odd sounds coming from your boiler, chances are, limescale build-up has created a hotspot on the heat exchanger.
  • Boiler lockouts: as mentioned, when limescale has built up on your heat exchanger, the water can overheat, which aside from making odd noises, will also cause your boiler to lockout if the temperature is too high. If your boiler is locking out frequently, it’s possible that the reason is an abnormally high temperature as a result of limescale buildup.
  • Water leaks: because the build-up of limescale can create hotspots on a heat exchanger, which can eventually lead to cracks, leaks are a common sign that something is not right. If you notice that your boiler is leaking water, it’s more than likely that the heat exchanger has cracked and is leaking somewhere.

Related Read: Is Your Boiler Making Noise? Here’s What It Means

How To Fix And Prevent Limescale Build-Up

To fix the limescale build-up problem, call an engineer who will check your boiler thoroughly and confirm that the issue lies with the limescale deposits. If you’ve caught the problem early, it’s possible that the heat exchanger can be repaired at a reasonable price. If if the problem is big (e.g. the heat exchanger is cracked in multiple places), it may be more cost-effective to invest in a new boiler.

To prevent limescale build-up problems in the future, install a limescale reducer which will collect the chalky deposit as it passes through the filter. You can also use a limescale silencer from time to time as it can help breakdown the deposits in the system.

Technician servicing heating boiler

Central Heating Sludge Build-Up

Central heating sludge is a combination of dirt and rust, or to be more precise, of dirt particles coming from the water in the system and iron oxide coming from inside the radiators and pipes. A build-up of central heating sludge will eventually create a blockage, which in turn, will create a fault with the heat exchanger.

Signs of sludge in your system

  • Odd noises: like limescale build-up, a build-up of heating sludge will often make a heat exchanger make odd noises. However, noises that the sludge build-up creates are different from kettling or whistling noises; these noises can often sound like tapping or scratching as the rust passes through the exchanger.
  • Boiler lockout: central heating sludge built-up can also cause your boiler to lockout because it restricts proper water flow, resulting in heated water not being able to travel around the heating system as quickly as it should. This can lead to an extreme change in boiler pressure, causing your boiler to lockout.
  • Dirty radiator water: aside from frequent boiler lockouts and strange noises, sludge build-up can also be recognized by dirty, almost black water coming out of the radiators when you bleed them.

How To Fix And Prevent Heating Sludge Build-Up

If you believe that heating sludge buildup is the cause of your boiler heat exchanger problems, call an engineer. They will inspect the component in detail and run a power flush to remove any sludge (you can also run a power flush yourself if you’re an experienced DIYer, but it’s always best to consult with an expert).

To prevent sludge buildup in the future, we recommend installing a magnetic system filter. You can also use a central heating inhibitor from time to time.

How Much Does It Costs To Repair Or Replace A Heat Exchanger?

Unfortunately, repairing or replacing a heat exchanger is often very expensive. So much so, that it rarely makes sense to spend money on repairs. To make matters worse, most damaged heat exchangers cannot be repaired at all but only replaced, so you can expect to pay anywhere between £300-£600 for a new component.

Thankfully, if your boiler is under warranty, you won’t have to pay to fix it. For this reason, it’s essential not to open the casing yourself; if you notice any issues, including heat exchanger faults, hire a professional so you can take advantage of your warranty.

If, on the other hand, your boiler is old, it may be more cost-effective to purchase a new unit with a long warranty period than to invest in an already deteriorating boiler.

How Long Does a Boiler Heat Exchanger Last?

The average life expectancy of a new boiler heat exchanger depends on various factors, but approximately this is 6-10 years for aluminium and 10-15 years for stainless steel (and a whopping 20 years or more for cast iron but finding a new and reasonably priced boiler with a cast-iron heat exchanger is next to impossible). This is why we always recommend purchasing from manufacturers that use stainless steel boiler heat exchangers –  they’re sturdier and more durable than aluminium, therefore they tend to last longer. Bear in mind that some causes of heat exchanger fault can be due to installation error, so it’s very important to hire a professional when replacing your heat exchanger.

Can a Boiler Heat Exchanger be Cleaned?

A boiler heat exchanger can and should be cleaned from time to time but we only recommend you do this with the help of a professional. An engineer should first thoroughly inspect the boiler heat exchanger so they can see if the problem can be fixed by simple cleaning. Bear in mind that some heat exchangers require special cleaning products; for instance, stainless steel exchangers should not be cleaned with hydrochloric acid.


  1. What is a heat exchanger in a boiler? – Viessmann