Heat exchangers [the ultimate guide] Re June 29, 2021

Heat exchangers [the ultimate guide]


We often take for granted that our boiler simply delivers hot water to our taps and radiators to keep our homes warm and toasty, but we don’t think about the important elements that allow our boiler to do this. Many are unaware that a heat exchanger is one of the most vital parts of our boilers. But what exactly does a heat exchanger do?

By the end of this blog, you will be fully clued up on exactly how a heat exchanger works and why it is one of the most important elements of your heating system.

What is a heat exchanger?

A heat exchanger does exactly what it says on the tin. They allow heat to be exchanged between two fluids or substances, typically water or gas, without letting the substances mix together. 

Heat exchangers can be found in all sorts of appliances and industries. For example, they can be found in power plants to recycle heat in the waste gases, as well as being used to heat swimming pools. Heat exchangers can also work in the opposite way, for example in refrigerators and air-conditioners, to draw the heat away from the room and keep compartments cool.

How do heat exchangers work?

Heat exchanges work by transferring heat from one area to another. When your boiler is ignited and begins to burn natural gases, the by-products (flue gases) enter and travel through the heat exchanger. Within the heat exchanger is a long, coiled pipe. The hot flue gases then heat the coiled pipe which then gradually heats the cold water surrounding it.

A heat exchanger must be used to stop the two substances from mixing, as this would prevent your boiler from working efficiently.


Types of heat exchangers

Without a heat exchanger, your boiler wouldn’t be able to heat the water that is distributed to your radiators and comes out of your taps, which is why a heat exchanger a really important part of your boiler. Essentially, without a heat exchanger, you would have no hot water to heating.

Stainless steel heat exchangers

Your heat exchanger must be made of strong materials so that it is long-lasting. Stainless steel is highly durable, making it a suitable material for this purpose. 

The benefit of using stainless steel heat exchangers is that there is less chance of corrosion. This means that your heat exchanger will last longer and maintain its efficiency, saving you money in the long run. 

Stainless steel heat exchangers can also withstand a higher flow rate, allowing your water to be heated quicker. This increased flow rate also minimises the risk of blockages within the exchanger from debris or rust, which can also increase its lifespan. 

In addition to this, stainless steel is self-cleaning and is a very lightweight material, which makes it ideal for use in a range of appliances that require water to be heated.

Aluminium heat exchanger

Aluminium heat exchangers are more tolerant to variable system quality (e.g. flow rate settings, air, and dirt) than stainless steel designs, as they allow for bigger waterways.

This means that they are well suited for a boiler replacement on an old system.

An aluminium heat exchanger will allow for a wider range of temperature differentials, typically 11–20 °C, meaning that the aluminium heat exchanger is perfect for older systems that are unable to reach a 20 °C differential without making system modifications.

Heat exchangers in non-condensing and condensing boilers

It is now law that all new boilers have to be condensing boilers, as they are much more efficient than non-condensing units. This is because condensing boilers having two internal heat exchangers, allowing them to capture and reuse the heat from waste gases.

What is the difference between a condensing and non-condensing boiler I hear you ask? Here’s the answer:


Non-condensing units have 1 heat exchanger. Cold water enters the heat exchanger to be heated up and then circulates around the home to radiators and the hot water tank.

Waste gases are sent outdoors via a flue, which can reach temperatures of up to 250°C, showing just how much heat is being wasted.

This isn’t a very efficient process, which is why condensing boilers have been made mandatory in the UK. They are not only better for the environment but also help save you money. You can thank us later 😉

Condensing Boilers

Condensing gas combi boilers have two heat exchangers.

To begin with, cold water enters the secondary chamber, absorbing heat from the flue gases that would otherwise be wasted by a non-condensing unit. The water then passes through the primary heat exchanger and creates hot flue gases.

This hot gas then travels into a secondary heat exchanger where more heat is released from the flue gas and water vapour begins to form. This change of state from water as a vapour to a liquid releases latent heat in the secondary heat exchanger, bringing the furnace to an even higher level of efficiency. This is why condensing boilers are known for being more efficient. 

As the water moves through your radiators, it’ll slowly begin to cool down. Your boiler is able to determine the temperature of the water. If it has cooled down too much, it’s sent back to the heat exchanger to be reheated. If the water is still hot enough, it is pumped around the heating system again.

How to clean a boiler heat exchanger

Even if your boiler’s heat exchanger is made from stainless steel which is self-cleaning, it may need an extra clean every now and then.

Dirt and debris from corroded radiator pipes can clog up your system. In some cases, you may find that your radiators are cold at the bottom or a pipe becomes clogged completely. 

You should contact a suitably qualified heating engineer who will be able to clean the heat exchanger for you. Engineers will often do this during your annual service. The engineer will locate the exchanger and apply water at high pressure to blast out any dirt or debris. You should make sure that this task is done every 12 months to keep your boiler working at its most efficient for longer.

Common heat exchanger faults

Similar to all 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 and 

If you feel like you’ve had enough of spending money constantly trying to fix your old boiler, then it’s time to think about buying a new one. WarmZilla can get you a boiler quote in 90 seconds!

Limescale build-up

Limescale is not usually an issue unless you live in an area with hard water. Limescale is a white, chalk-like substance that can cause great damage to appliances and pipework. If you have a build-up of limescale present in your water that flows through your heating system, this can cause your boiler to become less efficient, and eventually stop working. It is particularly troublesome for heat exchangers, as they are constantly exposed to 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? 

As mentioned, if you live in an area with hard water, then the chances are, you will be faced with issues at some point. Here are some of the most common signs of limescale build-up.

Please ensure that if you notice any of these signs, that you contact your local Gas Safe Engineer who may be able to assist you on what to do next. 

  • Whistling and kettling noises
    When limescale deposits build up in your boiler, they overheat, which can result in the production of steam and bubbling, causing a familiar whistling noise, referred to as kettling. 
  • Leaking heat exchanger
    This 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.

If you encounter one or a number of these signs, your boiler may already be on its way out. If you are looking for a new or replacement boiler, click here for a quote in minutes. 

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. 

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? We have a blog that covers reasons why your boiler could be making strange noises.

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 powerflush, or system cleanse can fix sludge. Not sure what a powerflush is? Don’t panic, we have this covered. 

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, your system likely has a sludge problem.

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 recommended that you’re heating system has a ‘flush’ every five years, however, if you suspect an issue, it’s better to call your Gas Safe Engineer to have a look. It’s better to be safe than sorry.  

An engineer can do the following steps 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

This is a great time to mention, that when purchasing a new or replacement boiler with WarmZilla, your boiler package will include the cost of a system cleanse, removing any dirt, debris or built-up from your heating system.

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.

You may find that boiler brands such as Worcester Bosch and Ideal offer an extensive warranty on their heat exchangers for reasons like we have discussed in this blog. If your boiler type is relatively new, it is likely to be under warranty and so any repairs will be covered. 

However, if your boiler is out of warranty, it may be better and more cost-efficient to buy a new boiler. If you’re interested in purchasing a new boiler, but you’re unsure what boiler you need, we have a blog on the best boilers to buy in 2021.

You can also take a look at our blog ‘what kW output you need for your home’. 

When purchasing with WarmZilla, you will receive a full system cleanse before your new boiler is installed, meaning all issues with poor-quality water and build-up should be resolved.

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

However, if you do decide to replace your heat exchanger, you are typically looking at a price range anywhere between £400 – £600. Remember that this does not include the cost of installation, which can depend on your local Gas engineer’s labour costs.

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