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Witten by Bernard Morgan (Engineer) & Adam Apperley (Marketing). Published: September 17, 2019. Updated: May 9, 2024

What is a heat exchanger in a boiler?

Heat exchangers in a boiler transfer heat produced from gas to water for your heating or hot water. A heat exchanger is the most important component in a boiler. In some ways, a heat exchanger’s role is like the engine in a car.

In the following article, we’ll discuss types of heat exchangers, materials and corrosion. We’ll also go through different boiler manufacturers currently selling in the UK.

Types of heat exchangers in your boiler

Heat exchangers can vary in appearance, but materials also differ across different manufacturers. Most manufacturers in the UK market use aluminium in their heat exchangers. Stainless steel is another popular material to use.

There are advantages and disadvantages for both.

Aluminium is cheaper to manufacture and lighter inside the unit. It also has a very high thermal conductivity. This means it transfers heat 15 times faster than stainless steel. However, aluminium is more prone to corrosion from magnetite sludge and other chemicals. This corrosion can cause a 10% drop in efficiency over a 10-year period.

Stainless steel is a harder material that is more difficult and expensive to manufacture. Stainless steel also doesn’t lose efficiency over time, and, in fact, can get more efficient. It is a lot less prone to corrosion and is best used with normal water from the mains.

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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

Who has the best heat exchanger?

The table above explains the differences between materials used in heat exchangers. But claiming that ‘stainless steel is the

Viessmann's patented Inox-Radial stainless steel heat exchanger

best’ isn’t enough. You also need to know if the steel from one manufacturer is better than another.

Manufacturers may offer stainless steel for some of their boilers. But a low-grade stainless steel won’t give your boiler the longevity of a high-grade stainless steel.

Some of the brands listed provide boilers with aluminium or stainless steel. However, only Viessmann offers high-quality stainless steel across their whole range. Viessmann’s stainless steel is only one grade down from surgical-grade steel.

Popular brands sold in the UK

Viessmann

Viessmann has patented their Inox-Radial stainless steel heat exchanger and manufacture it in-house. This keeps costs down and ensures quality assurance.

All boilers are placed under high thermal stress every day. So heat exchangers need to be manufactured with the best materials and to the highest quality. Viessmann use 10.5% chromium and 1.2% carbon by mass stainless steel mix. This provides the best corrosion resistance at the widest range of pH levels. Aluminium doesn’t come close.

The graph below shows performance over time. Aluminium (red) drops straight away, reducing heat output at the same gas input. Viessmann’s stainless steel heat exchanger becomes more efficient over the first thousand hours of use.

heat exchanger comparison

Worcester Bosch/Ideal/Baxi/Vokera

Popular brands that use aluminium heat exchangers

Some of the boilers made by the above manufacturers may have either aluminium or stainless steel. Although stainless steel is the superior material, aluminium isn’t bad. Aluminium requires special inhibitor chemicals for maintenance. 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 is vital to extending the longevity of component
  • Aluminium corrodes via a build-up of aluminium oxide
  • Erosion increases if flow rates are higher
  • Reduced lifespan of the component itself

It’s true that stainless steel is more expensive to source and manufacture. But, it can make a big difference in the longevity of your boiler.

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What happens if I go aluminium?

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.

limescale shower

Hard water and limescale

All areas of the UK measure ‘water hardness’. This refers to the level of dissolved calcium and magnesium compounds found in the water. This contributes toward a build-up of limescale. If you’re based in a ‘hard water area’ there’s a good chance limescale will build up on your heat exchanger.

To prevent limescale, you can keep an eye on your boiler’s performance. You might notice a ‘kettling’ or ‘whistling’ noise from the boiler. This happens when steam bubbles expand from the higher temperatures. This occurs from limescale spots forming on the heat exchanger.

Limescale build-ups can cause overheating. So boilers are fitted with a thermistor. This sensor detects if the water gets too hot. It then responds by changing the voltage that controls the temperature. But, this may only protect your boiler for a short time.

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 leaks will start showing.

How can I prevent limescale?

As with everything, discovering any kind of fault is never ideal. A replacement heat exchanger may cost upwards of £1,000. This may mean replacing the boiler is a better option as you will get a new boiler with a warranty.

The best option is regular servicing of your boiler and central heating. They will be able to keep an eye on your system and advise what can be done to reduce limescale. An engineer may even be able to prevent build-up in the first place.

Beyond checking the heat exchanger, you can also put various preventative measures in place.

  • Limescale silencer installation – 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 cut the problem before it gets worse.
We hope that this will help you make an informed decision on your new boiler. Here at Boilerhut we recommend the Viessmann Vitodens range. When installing them we follow the manufacturer’s guidance. This will include water and magnetic filters, the flue and a thermostat.

New boilers aren’t the cheapest thing in the world. We’re here to help.

We suggest getting a few quotes for boilers online, including with us, so you get an idea of pricing. We can then call you back to discuss the finer points of the quote to ensure it’s all correct.

We’re not about hard sales; we’re about informed decisions. We would rather you made the right choice for you elsewhere than the wrong choice with us.

To get a quote, please follow the online quote system or call us on 029 2009 9898. Quotes are free and have no-obligation attached.

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