Category - Boilers
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Will Scholfield, Engineer

31 Oct : Updated 22 Mar ● 7 min read

What is a boiler heat exchanger?

Whilst you might not have heard of a heat exchanger, you probably own more than one appliance that uses one. Found in white goods such as refrigerators and air conditioners, there are heat exchangers inside your boiler, too. And, without a fully functioning boiler heat exchanger, you’d be hard-pressed to find heating and hot water in your home. 

Therefore, knowing what it is, how it functions and what to look out for if it’s faulty is important to stay ahead of the game. In a nutshell, heat exchangers work to transfer heat, making them a crucial component of any boiler. Found commonly in combi boilers, system boilers and standard boilers, heat exchangers come in pairs - one for central heating and one for hot water. 

This article will provide you with information about what a heat exchanger is, why it may be faulty, what to look out for and whether it’s time to upgrade your boiler.

How does a heat exchanger work?

As we mentioned before, a heat exchanger is a crucial element in a boiler. Without one, your water would simply stay cool, and your boiler wouldn’t be able to do its job. But how does a heat exchanger work exactly? And why are they so important? 
Heat exchangers are clever components, and, as the name suggests, their primary objective is to exchange energy (heat) to pass from one fluid to another. This is achieved by heating gas which then turns to hot liquid, without the two particles ever touching. 

The heat exchanger, which sits inside your boiler, is made up of a coiled pipe structure which has cold water running adjacent to it, and it works like this: 

  1. When you turn the heating on, gas inside the boiler will start to burn
  2. Heat rises as the gas burns to meet the heat exchanger
  3. As the water circulates, heat is exchanged between the burning gas to the cold water, which runs adjacent to the structure 

This process produces hot water for both water and heating throughout the home by allowing the efficient transfer of heat from burning gas to cold water. Without a heat exchanger, a boiler cannot produce hot water. 

How does a heat exchanger work on a combi boiler?

Short for combination, combi boilers combine domestic heating and hot water - all from the same system. Because of this, the heat exchanger that sits inside a combi boiler is a little bit different to a standard or system boiler. 

In fact, the heat exchanger in a combi boiler is made up of two parts. The primary heat exchanger controls the central heating, whereas the secondary heat exchanger is responsible for hot water. 

The primary heat exchanger works by heating and recycling water that travels through the pipes and into radiators around the home. As for the secondary exchanger, known as the secondary plate heat exchanger - it works by heating up the water that flows through the taps. 

Types of heat exchangers

Due to the nature of their role within an already high-functioning environment, heat exchangers are put under a lot of stress. That’s why they need to be made from strong, durable and corrosion-resistant materials. Commonly, they are made using aluminium or steel. 

Before the introduction of condensing boilers, non-condensing models were the only option on the market.

Non-condensing boiler heat exchangers

Less popular due to their lower efficiency, non-condensing boilers have a single heat exchange chamber which is where the gas is stored. By law, all new A-rated boilers are now fitted with condensing heat exchangers.  

Condensing boiler heat exchangers

Carved from a much more efficient design, condensing heat exchangers operate, like non-condensing, with a coiled pipe. However, it can extract much more energy from the gases passing over it, the difference being the length of the coiled pipe. 

Condensing vs non-condensing boilers

Because they are so much more efficient than non-condensing units, all new boilers are required by law to be condensing. This is because condensing boilers can store and reuse heat from waste gases due to the two internal heat exchangers.

The length of the pipe in a condensing heat exchanger is longer than in a non-condensing, thus making it more energy efficient. In fact, a typical condensing boiler runs at over 90% efficiency. Whereas, non-condensing only reaches up to 78% efficiency

Due to this, in 2005, the UK government introduced a series of condensing boiler regulations to help impact the reduction of carbon emissions. These regulations state that as of 2005, it is no longer legal to install non-condensing boilers in homes in the UK.

Common heat exchanger faults

If your boiler is making an unfamiliar noise, locking out or leaking - then you might have a faulty heat exchanger. Luckily, boiler heat exchanger lifespans can be improved with proper maintenance, resulting in more effective and economical heating. Below are some of the most common heat exchanger faults:

1. Natural wear and tear 

Similar to most appliances and white goods found in homes, over time, your boiler and its parts (including the heat exchanger) will show natural signs of wear and tear. Modern stainless steel or aluminium heat exchangers will typically last anywhere between 15 to 25 years, provided your boiler is well looked after and regularly serviced.

2. Limescale build-up 

Minerals in the water that is pumped into the heating system cause limescale to form - as a result, it can cling to pipelines, towel rails, and radiators. These minerals are typically found in moderate to hard water areas.

So, if you have the pleasure of waking up to South-coast sea views, then you may not need to worry about limescale buildup on your heat exchanger. However, if you live further north in an area with moderate to hard water, then chances are you could fall victim to this common heat exchanger fault. 

4. Central heating sludge 

As nasty as it sounds, central heating sludge can be more common than you think. Sludge is created by the mixing of two very common materials found in central heating systems. These are dirt particles and bits of rust (iron oxide) - which typically break off from radiators, the pipework and joints over time. 

Because of this, older systems are more likely to have a build-up of central heating sludge. Luckily, there are ways to remove and ‘flush’ the sludge from a system, and preventative measures, too. Magnetic filters are an effective way to help prevent central heating sludge buildup.

Other telltale signs of a faulty heat exchanger are: 

Repairing or replacing a broken heat exchanger

Sometimes, your heat exchanger will be beyond the point of repair; in that case, you’ll have to bear the cost associated with replacing it. 

Typically, a new heat exchanger will set you back anything from £1,500 to £3,500 - if it’s looking like it’s going to be a costly replacement or repair, then you might need to consider a new boiler. However, always check your manufacturer’s warranty (if it’s still valid), as repair costs may be covered. The same goes for if you have boiler insurance or boiler cover - check what’s covered in your plan before you fork out for repairs.

When it comes to the world of hot water and heating, there’s a fair amount of jargon to get your head around. Luckily, the team at BOXT are experts in our field and aim to provide you with the most up-to-date and accurate information to make sure your property or home is getting the best service possible and staying warmer for longer. 

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