You may take it for granted that your boiler is able to heat water up and then deliver it around your home, either through your radiators or to your taps. You might not, however, have thought about how it achieves this. The heat exchanger is the key. Without this device, your boiler wouldn’t be able to warm up your water. So what exactly is a heat exchanger and how does it work in your boiler?

What is a heat exchanger?

A heat exchanger does precisely what its name describes - it allows heat to be exchanged between two fluids or substances, usually water or gas, without letting the substances mix together.

Within the heat exchanger is a long, coiled pipe. This pipe is surrounded by a substance that can be heated, such as cold water. A hot substance, such as gas or water, is fed through the pipe and, as it moves, it gradually heats the surrounding water. A heat exchanger doesn’t allow these two substances to combine, as this wouldn’t work as effectively - the water can be heated up much faster than if hot water was simply added to cold water.

Heat exchangers can be found in all sorts of appliances and industries. They’re used to recycle heat in the waste gases at power plants and to heat swimming pools. They can also work in the opposite way and cool something down. Refrigerators and air-conditioners contain heat exchangers which draw the heat away from the room or compartment to keep it cool. 

Heat exchangers are most commonly found in boilers. They allow your boiler to heat the water that is then pumped through your radiators to warm your home. Let’s take a closer look at how a heat exchanger works in your home’s combi boiler.

How does a combi boiler heat exchanger work?

A boiler works by using hot gas to heat water - and a heat exchanger is what allows this to happen.

As the gas is heated, it begins to rise. When it rises, it reaches the heat exchanger and is pushed through the coiled pipe. Cold water is surrounding the pipe and, as the hot gas moves through it, it gradually heats the water, which is then ready to be supplied to your radiators and taps.

It’s now a legal requirement to have a condensing boiler instead of a non-condensing boiler fitted when replacing a boiler or fitting a new one for the first time. This is because the former is much more efficient.

A non-condensing boiler only has one heat exchanger. While this may not necessarily seem like a bad thing, the waste gases that exit the boiler flue could reach up to 250°C. This is wasted heat that could be recycled and used in the system to reach maximum efficiency. A condensing gas boiler is able to recycle this heat from the waste gases and use it to heat the water.

Condensing gas combi boilers have two heat exchangers. To begin with, the water is heated in the primary exchanger as normal. As hot waste gases are produced, the water that has returned from its circuit around the radiators can be pushed into the secondary heat exchanger, which only uses waste gas to heat the water. Once it’s been heated slightly in this heat exchanger, it can return to the primary one to become even hotter. In this way, the waste gas is being used for something productive, which hugely increases the boiler’s efficiency. The waste gas that exits from a condensing boiler’s flue can be up to 200°C cooler than the gases from a non-condensing boiler. This is why condensing boilers have up to 98 per cent efficiency.

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. 

What boilers have stainless steel heat exchangers?

Without a heat exchanger, your boiler wouldn’t be able to heat the water that moves around your radiators and comes out of your taps. This makes a heat exchanger a really important part of your boiler. If it breaks, you won’t have any heating or hot water. Therefore, it’s imperative that it’s made out of a strong material that is long-lasting. Stainless steel is one of the best materials for this purpose.

There are many benefits of using stainless steel for the heat exchanger. First, stainless steel offers impressive resistance against corrosion. This means that your heat exchanger is more likely to last for longer and will maintain its efficiency, which could save you money on your heating bills.

Second, stainless steel heat exchangers can withstand a higher flow rate than exchangers made from other materials. If more gas or water is able to pass through at a faster rate, your water can be heated up more quickly. This increased flow rate also minimises the risk of blockages within the exchanger from debris or rust. This could again increase its lifespan.

Finally, 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.

All of Viessmann’s condensing boilers have stainless steel heat exchangers so you can be assured that your boiler will run at maximum efficiency. 

How to clean a boiler heat exchanger

Even if your boiler’s heat exchanger is made from stainless steel, it may need cleaning 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. If you’d prefer, you can ask them to do this during your boiler’s 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.

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