What is a heat exchanger in a boiler?

Popular brands sold in the UK

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.

Worcester Bosch/Ideal/Baxi/Vokera

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.

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.

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.