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Last Updated on May 1, 2023

What is a heat exchanger in a boiler?

The purpose of the heat exchanger in your boiler is to transfer the heat produced from gas into water. This hot water then moves through the pipework and into your radiators. To put it simply, a heat exchanger is the most important component in the boiler. In a lot of ways, a heat exchanger’s role is similar to the engine that’s inside your car!

Hopefully with this explanation you’ll have a better idea of what a heat exchanger is.

In the following article, you’ll see examples of heat exchangers used by boiler manufacturers that are currently operating and selling in the UK. You’ll also learn about which materials allow for a longer lifespan.

For further information, you can read detailed comparisons of several brands in the heating arena:

Types of Heat Exchangers in your Boiler

Heat exchangers can vary in appearance, but materials also need to be considered. Most will either be aluminium or stainless steel, but stainless steel has much better corrosion resistance, so your boiler is better protected.

For aluminium heat exchangers, there are also disadvantages that come with it. While they initially have strong thermal conductivity (heat transfer), they are more prone to corrosion from magnetite sludge and other chemicals incorrectly added to the system. As well as this, they cannot be used with softened water.

REMEMBER: Based on the above information, any damage to the heat exchanger may cause your warranty to be void. As a result, the manufacturer may decide not to replace the component.

Which Material is Best?

In order to compare UK-based manufacturers, it’s good to know what material is used for the heat exchangers.

In the following table, you can see the benefits that each material provides, giving you a much better idea of what a high quality boiler should be:

	Viessmann	Ideal	Baxi	Vokera	Vaillant
Material	Stainless steel: All Models	Aluminium: Logic Max range (Combi, System or Heat-only) Logic+ range (Combi, System or Heat-only) Stainless Steel: Ideal Vogue Gen 1 (Combi) Ideal Vogue Gen 2 (Combi) Ideal Vogue Max (Combi)	Aluminium: Heat-only Boiler range Stainless Steel: Combi Boiler range System Boiler range	Aluminium: All domestic boilers (Main Heat Exchanger)	Aluminium: Ecofit Pure Range Stainless Steel: Ecotec Plus range Ecotec Pro Range Ecotec Exclusive
Benefits	Stainless Steel: Patented design High corrosion resistance on wide range of pH levels No need for extra maintenance fluids Compatible with plain, clean water Very clean internally Operates effectively at high flow rates	Stainless Steel: High corrosion resistance No need for extra maintenance fluids Compatible with plain, clean water Very clean internally Operates effectively at high flow rates Aluminium: Low manufacturing costs Good initial thermal conductivity Lightweight	Stainless Steel: High orrosion resistance No need for extra maintenance fluids Compatible with plain, clean water Very clean internally Operates effectively at high flow rates Aluminium: Low manufacturing costs Good initial thermal conductivity Lightweight	Aluminium: Low manufacturing costs Good initial thermal conductivity Lightweight	Stainless Steel: High corrosion resistance No need for extra maintenance fluids Compatible with plain, clean water Very clean internally Operates effectively at high flow rates Aluminium: Low manufacturing costs Good initial thermal conductivity Lightweight

Viessmann

Ideal

Baxi

Vokera

Vaillant

Material

Stainless steel:

All Models

Aluminium:

Logic Max range (Combi, System or Heat-only)
Logic+ range (Combi, System or Heat-only)

Stainless Steel:

Ideal Vogue Gen 1 (Combi)
Ideal Vogue Gen 2 (Combi)
Ideal Vogue Max (Combi)

Aluminium:

Heat-only Boiler range

Stainless Steel:

Combi Boiler range
System Boiler range

Aluminium:

All domestic boilers (Main Heat Exchanger)

Aluminium:

Ecofit Pure Range

Stainless Steel:

Ecotec Plus range
Ecotec Pro Range
Ecotec Exclusive

Benefits

Stainless Steel:

Patented design
High corrosion resistance on wide range of pH levels
No need for extra maintenance fluids
Compatible with plain, clean water
Very clean internally
Operates effectively at high flow rates

Stainless Steel:

High corrosion resistance
No need for extra maintenance fluids
Compatible with plain, clean water
Very clean internally
Operates effectively at high flow rates

Aluminium:

Low manufacturing costs
Good initial thermal conductivity
Lightweight

Stainless Steel:

High orrosion resistance
No need for extra maintenance fluids
Compatible with plain, clean water
Very clean internally
Operates effectively at high flow rates

Aluminium:

Low manufacturing costs
Good initial thermal conductivity
Lightweight

Aluminium:

Low manufacturing costs
Good initial thermal conductivity
Lightweight

Stainless Steel:
- High corrosion resistance
- No need for extra maintenance fluids
- Compatible with plain, clean water
- Very clean internally
- Operates effectively at high flow rates
Aluminium:
- Low manufacturing costs
- Good initial thermal conductivity
- Lightweight

Who has the best heat exchanger?

As you can see from the table, there’s quite a difference with how each manufacturer uses materials for their heat exchanger. However, simply claiming that ‘stainless steel is the best’ isn’t enough. You also need to know if the steel from one manufacturer is better than another.

Manufacturers such as Vaillant or Baxi may very well offer stainless steel for some of their boilers, but a low grade material still won’t give your boiler the longevity you’re looking for if the quality isn’t high enough.

As you can see from the table, there are a few differences in the materials, but simply claiming that ‘stainless steel is the best’ isn’t enough. You also need to know why.

Some of the brands listed provide boilers with aluminium OR stainless steel, but there’s only one manufacturer among them that uses a superior stainless steel consistently across their entire boiler range, and that’s Viessmann.

Accepting only top quality materials, the stainless steel used for the Viessmann boiler range is only one grade down from surgical steel, making it an easy choice for people that a want a boiler designed with the customer in mind that can provide excellent efficiency.

Popular brands sold in the UK

Viessmann have patented their own Inox-Radial stainless steel heat exchangers, so they can keep costs down and continue developing and manufacturing high-quality components.

Their boilers are put under thermal stress every day, so only the best materials are used. This is why their heat exchangers have 10.5% chromium content and 1.2% carbon by mass. Because of this composition, their boilers have increased corrosion resistance at a wide range of pH levels. It’s easy to see how stainless steel is the preferred option for your heat exchanger. No other material comes close to this.

For a detailed, visual comparison of the performances of stainless steel and aluminium heat exchangers, take a look at the graph below:

As you can see, even though aluminium starts off well, it does eventually drop in efficiency. So you have to ask yourself… is it worth it?

Viessmann – Warranty

For their heat exchangers, Viessmann offer a 10-year warranty. Along with high-quality material and design, this component helps improve the efficiency of the boiler itself. In some cases such as the Vitodens 200-W, this can even be improved to up to 98%.

Please consider that this guarantee is only applicable if the following conditions are met:

Combustion settings are set correctly
You have an annual checkup arranged

Worcester Bosch/Ideal/Baxi/Vokera

Even though some of these brands offer stainless steel heat exchangers as well as aluminium, it’s important to recognise the benefits and detriments of aluminium and how it compares.

They also require special inhibitor chemicals for maintenance. However if used incorrectly, it can make it difficult to clear out aluminium oxides that cause further damage.

Key points to consider with aluminium heat exchangers:

Lower manufacturing costs compared to stainless steel
Treatment of pH levels vital to extending longevity of component
Aluminium corrodes via a build up of aluminium oxide
Erosion increased if flow rates are higher
Overall reduced lifespan of component itself

It’s true that stainless steel is more expensive to source and manufacture. However, what more people need to made aware of is how much of a difference it can make in the long run for your boiler’s longevity. Aluminium has been known to rust and corrode as it breaks down, which is something that’s not considered as much of a concern for stainless steel. As mentioned before, the material quality used in Viessmann heat exchangers is only one grade away from surgical steel, for example.

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What happens if I go cheap?

Choosing the right manufacturer is important, as some companies will go the extra mile to make sure that the components inside your boiler are of the highest quality. This is made even more important when you realise what you end up with if you go for a sub-par, cheap option. Using the information found in this article, we can see how the right material can extend the longevity of your boiler.

All areas of the UK are measured in terms of ‘water hardness‘. This refers to the level of dissolved calcium and magnesium compounds found in the water, which directly contributes toward a build-up of limescale.

If you’re based in a ‘hard water area’ such as Norwich or Bristol, then there’s a good chance limescale will build up on your heat exchanger.

To prevent this, you need to keep an eye on your boiler’s performance. You might notice a distinctive ‘kettling’ or ‘whistling’ noise coming from the boiler. This happens because of steam bubbles expanding from from the raised temperature, which occurs from limescale spots forming on the heat exchanger.

Because limescale build-ups cause overheating, your boiler will be fitted with a thermistor. This component detects if the water gets very hot, and responds by changing the voltage that controls the temperature. However, a build up of limescale can make this more difficult, because it forces the water supply to raise in temperature, so your thermistor isn’t getting a reading that’s 100% accurate.

However, if your heat exchanger is too old or has taken too much damage, then it may be prone to cracks. If this is the case, keep an eye on the bottom of the boiler – that’s where the leaks will come from.

How can I prevent limescale?

As with most components, discovering any kind of fault is never ideal. A replacement heat exchanger may cost upwards of £500, which doesn’t make a huge amount of sense if your old boiler continues to cause problems.

The best thing you can do at this point is to have a professional engineer help. If the problem is caught early, they may be able to help without costing you an arm and a leg.

Beyond checking the heat exchanger itself, you can also put various preventative measures in place, including:

Having a limescale silencer installed – Designed to reduce ‘kettling’ and ‘whistling’ noises while improving heat transfer efficiency.
Fit a limescale reducer – Unlike the silencer, this works to collect limescale as it passes through, helping to eliminate the problem before it gets worse.

What is a heat exchanger in a boiler?

Types of Heat Exchangers in your Boiler