What Is A Heat Exchanger In A Boiler? A Clear Explanation
Key Takeaways
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UK citizens rely on their home’s boilers to provide instant hot water and on-demand heating. But how does your home’s boiler achieve this immediate convenience? A short answer is via its heat exchanger.
Key Takeaways
- What is a boiler heat exchanger?
- How does a heat exchanger work?
- What are the differences between non and condensing boilers?
- What types of heat exchangers are there?
- Signs that your boiler’s heat exchanger is not well
- The importance of servicing and cleaning
- It might be best to replace the entire boiler
Your boiler’s heat exchanger is one of its most critical components. Ensuring it functions efficiently can prevent costly repairs or replacements. A new heat exchanger can cost between £400 and £600, but if your boiler is still under warranty, you may be able to get it replaced for free.
Be aware there are bound to be terms and conditions you must abide by.
Also, at the price point for a new heat exchanger, most Gas Safe registered engineers will recommend replacing older boilers if this part goes.
But what is a heat exchanger?
What Is A Boiler Heat Exchanger?
As the name suggests, a heat exchanger allows energy (in this case, heat) to move between elements (gas to liquid) without mixing them.
In the case of your home’s boiler, the combustion gas heats the water circulating through the system.
There are multiple types of heat exchangers, but generally, they all work with the same principles.
How A Heat Exchanger Works:
Step 1: Heat Transfer
Fuel is burned to generate heat in your home’s boiler. Gas boilers are the most popular heating system in the UK, with 75% of residential heating energy consumption being gas.
The heat is absorbed by the boiler’s exchanger, which is usually made from a metal, such as:
- Aluminium
- Stainless Steel
- Or, in older models, cast iron
Metal is an excellent conductor of heat.
Step 2: Heat The Fluid
The heat exchanger is in contact with the water (or fluid) circulating through the system that needs heating.
As the exchanger heats up, it transfers this heat to the water, raising its temperature. In steam boilers, this process produces steam by boiling the water.
The efficiency of a boiler depends significantly on how effectively the heat exchanger transfers heat from combustion to the water.
So, what happens inside a combination (combi) boiler as it does two jobs in one?
Combi Boilers and Heat Exchangers
A combi boiler typically has two heat exchangers:
- The primary heat exchanger heats water circulating through your radiators, consistently providing warmth.
- The secondary heat exchanger delivers hot water to your taps, baths, and showers.
Since 2005, the installation of condensing boilers has been mandatory. But what’s the difference between a non-condensing and condensing boiler, and what does it do with heat exchangers?
In 2005, the UK’s Building Regulations updated performance standards for boiler replacements moving forward.
By 2016, all UK boilers were required to comply with the Energy Related Products methodology (ErP) rating system used in the EU.
With the introduction of this new ErP rating, the requirement was that all gas boilers installed into existing systems in England have to be at least 92%.
Under these regulations, gas boilers installed in existing systems in England must have an efficiency rating of at least 92%. Only condensing boilers met this requirement.
Understanding why non-condensing boilers don’t meet this efficiency requirement is down to how they work.
Non-Condensing Boilers
A non-condensing boiler has only one heat exchanger, and when the fuel is burned, the waste gases are simply vented out of the flue. However, these expelled gases can still be incredibly hot.
Research shows that the minimum flue gas temperature expelled from boilers is between 120 and 130 °C but can easily reach 250°C. In non-condensing boilers, this heat is simply wasted, which can mean a loss of 10% to 30% of heat energy.
Condensing Boilers
A condensing boiler uses a heat exchanger to capture and use this escaping energy. Depending on your boiler, it could have one or two heat exchangers.
Another system that selected condensing boilers could use is a Flue Gas Heat Recovery System, which is designed to recycle the heat from the wastes gases.
As you can imagine, many brands use different technologies in their boilers. This includes various types of heat exchangers, which can vary widely.
Types Of Heat Exchangers
Heat exchangers are typically made from metals. The two most common materials used today are:
Aluminium Heat Exchangers
As a metal, aluminium is lightweight and has a high thermal conductivity.
Aluminium is also flexible, corrosion-resistant, and cheaper than other metals. Smaller boilers usually use aluminium heat exchangers.
Stainless Steel Heat Exchangers
While more expensive and heavier than aluminium, stainless steel heat exchangers are more durable and corrosion-resistant than their counterparts.
Also, stainless steel heat exchangers usually last longer than aluminium ones.
As a side note, you may find a cast-iron heat exchanger in very old boilers, but these are now rare.
Heat exchangers also come in a selection of designs.
An older cast-iron heat exchanger was a lump of heated metal in the middle of the water flow. While highly durable, it wasn’t as efficient in transferring heat as more modern designs.
These new modern designs include:
Tubular Heat Exchanger
Tubular heat exchangers use a series of metal tubes laid out in a coiled or straight line and enclosed in a sealed-off compartment.
When heated by the combustion source, gases flow through the tubes, transferring the energy to the circulating water source. The design of these tubular heat exchangers allows for efficient heat transfer by creating a large surface area.
Plate Heat Exchanger
A plate heat exchanger is usually more compact than a tubular layout. It consists of many layers of thin, flat metal plates stacked closely on top of each other, creating a large surface area for heat exchange.
Usually, plate heat exchangers will be used in combi boilers and are incredibly efficient.
It’s important to remember that no matter what type of heat exchanger is in your boiler, they are all vulnerable to failure if not appropriately maintained.
But what signs there’s something wrong with your boiler’s heat exchanger?
Signs That Your Heat Exchanger Needs Help
A Gas Safe register engineer should check your home’s boiler every year. By ensuring your boiler is serviced annually, you also lower the chances that problems with its heat exchanger don’t happen.
There are two definite signs your heat exchanger needs help.
Kettling Noise
When your home’s boiler makes any strange noise, it can be extremely alarming. A high-pitched squealing, like a stovetop kettle, often indicates an issue.
This happens when water flow is reduced, causing the remaining water to overheat, boil, and release steam, producing noise.
This is often due to limescale buildup.
Another noise that can indicate a system blockage is a tapping sound. This blockage is usually caused by sludge buildup in your central heating system.
Leaks
While noises from your boiler will indicate a current blockage, leaks are caused by neglected or old heat exchangers.
If water is leaking from the bottom of the boiler, it can be a sign that your heat exchanger is cracked.
A leaking boiler unit requires a Gas Safe registered engineer to repair or replace it.
Dirty Radiator Water
When bleeding your radiator, if the water is black or discoloured, it clearly indicates a sludge build-up in your heating system.
A build-up of sludge in your system will eventually damage your heat exchanger.
It’s essential to ensure your home’s central heating system is clear from debris, as blockages will shorten the lifespan of your boiler’s heat exchanger.
Cleaning Or Flushing Out Your Boiler’s System
A clean boiler system will prolong its life.
In turn, it will ensure that your heat exchanger works as efficiently as possible for longer.
When you have your annual service check from a Gas Safe registered engineer, they will add a limescale or sludge cleaner to the water supply. This will ensure your system’s water is clean and clear for longer.
Another option is to install a system filter to remove debris from the system.
However, it is strongly recommended that your home’s boiler system be flushed out every five years. While some may say you can flush out your own system, we strongly recommend calling in a professional.
But what if it’s too late, and the heat exchanger completely breaks?
Replace Your Boiler
Before replacing your boiler, you should review whether you can claim a new heat exchanger under a valid warranty.
If the warranty has lapsed, it is often best to replace the entire boiler. No matter who you are, this is a bitter pill to swallow.
However, when you buy a new boiler, you get numerous benefits from installing a more efficient unit with a new and valid warranty.
If you have any questions about heat exchangers or buying a new boiler, feel free to message us.
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