What is a Heat Exchanger in a Boiler?

Ideal Heat Solutions use heat exchangers to break up the system into different pressure zones for example high and low pressure.

A heat exchanger is a device that transfers heat from one fluid to another, without the fluids mixing. The fluids can be gases, liquids, or a combination of both. Heat exchangers are commonly used in heating, cooling, and refrigeration systems, as well as in industrial processes and power generation.

The basic principle of a heat exchanger is to use a conductive material, such as metal, to transfer heat from one fluid to another, without the fluids mixing. The two fluids flow through the heat exchanger in separate channels, with the heat transfer occurring through the walls that separate them. The heat is transferred from the hotter fluid to the cooler one, until they reach the same temperature.  Heat exchangers come in many different shapes and sizes, depending on the specific application. Some common types include shell-and-tube, plate-and-frame, and air-to-air heat exchangers.

In a shell-and-tube heat exchanger, the two fluids flow through separate channels, with one flowing through tubes and the other flowing through a surrounding shell. Heat is transferred between the two fluids through the walls of the tubes.  In a plate-and-frame heat exchanger, the two fluids flow through a series of parallel plates, with the heat transfer occurring through the plates themselves. In an air-to-air heat exchanger, the two fluids are both gases and flow through separate channels, with the heat transfer occurring through the walls that separate them.

In a combi boiler system, which combines central heating and hot water within the same unit, there are 2 heat exchangers. A primary heat exchanger for the central heating and a secondary heat exchanger for the hot water.  The primary heat exchanger heats and recycles the water that goes through the pipes into the radiators and the secondary heat exchanger heats up the hot water that goes into your taps. The primary heat exchanger is like an engine in a car, the efficiency and longevity of the heating system depends on the quality of the internal components.  A combi boiler system blasts its primary heat exchanger with flames at around 700 to 900 degrees to provide central heating. Radiators will heat gradually over time but the inside of a boiler is like a furnace – constant, extreme thermal stress on the heat exchanger on a daily basis and almost 24/7 during winter.  This is why it is critical that a heat exchanger is made of a reliable, durable metal of the highest quality or else it can corrode and lose its efficiency over time, for example, Stainless Steel and Chromium Alloy as they are extremely resistant to corrosion.

Heat exchangers are essential for many heating and cooling systems, by transferring heat from one fluid to another, heat exchangers help to improve energy efficiency, reduce operating costs, and improve overall system performance.

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.

What does a heat exchanger do on a combi boiler?

A combi boiler (or combination boiler) provides both central heating and hot water within the same system. Without a working heat exchanger in your combi boiler, you won’t receive any hot water or heating.

Usually, a combi boiler will have two heat exchangers – one for central heating and one for hot water.

• Primary heat exchanger. The primary heat exchanger is used to heat the radiator water, which travels to the radiator and then back to the boiler. It is consistently warmed as it cycles through the heating system.
• Secondary plate heat exchanger. This is heated by the primary heat exchanger. When a hot water tap is turned on, water is diverted from the Primary Heat Exchanger into the secondary heat exchanger until the hot water tap is turned off.

The different types of heat exchangers

As you can imagine, heat exchangers must sustain a high amount of stress when intense heat is consistently applied to the material. The most common materials you will find are stainless steel and aluminium:

• Aluminium heat exchangers are lightweight and have a high thermal conductivity. The material itself is highly flexible and can be cheaper than stainless steel to install. Aluminium heat exchangers are often smaller than stainless steel types. The Aluminium is designed to be resistant to corrosion over the design life of the product
• Stainless steel heat exchangers are desired for their durability and very high resistance to corrosion, meaning they often maintain a longer lifespan than their aluminium counterparts. The stainless steel includes a high amount of chromium in order to be highly corrosion resistant which can impact the durability of the material.

Faulty boiler heat exchanger symptoms

Whether the material is aluminium, stainless steel or otherwise, we recommend that your heat exchanger is checked every 12 months by a gas safe engineer to identify issues before they reach urgency. Without a professional review, it can be difficult to know the ‘health’ of your heat exchanger, though there are some clear signs we recommend looking out for.

• Noises coming from boiler – If you have started to notice strange noises coming from your boiler, this may be indicative of a faulty heat exchanger. Listen out for high-pitched whistling noises, also known as ‘kettling’. This happens due to limescale build-up and is a clear sign your exchanger needs to be cleaned. Tapping noises may be a sign of central heating sludge, which causes pieces of rust and debris to pass through the system and cause a blockage.
• Leakages – Noises from the boiler can be a sign of a build-up of debris in the heat exchanger. Without being serviced, this could eventually lead to your heat exchanger leaking. If your boiler is quite old and water is constantly leaking from the bottom of its casing it could be a sign that the heat exchanger is faulty, meaning a replacement may be required. On modern boilers it is sometimes harder to identify a cracked or leaking heat exchanger. This is best left to a qualified professional to identify any issue.
• Dirty radiator water – Dirty radiator water can cause a blockage and, unfortunately, serious damage to your boiler. You’ll be able to see this by bleeding your radiators, where the water will appear black or discoloured. This is caused by central heating sludge, a collection of rust, debris and residue from copper pipes and radiators.

How to clean your heat exchanger

Keeping a clean and maintained exchanger will prolong the life of your boiler. This rings true for whichever boiler or type of exchanger you have, as there is always the risk of dirt or debris from your radiators.

You’ll need to get in touch with a Gas Safe registered engineer to clean the device for you. Never remove a heat exchanger yourself, as it is not safe to do so and you could cause unnecessary costly damage to your boiler.

• Limescale – Whilst we strongly recommend you do not remove or clean the device yourself, you can take steps to reduce the risk of limescale build up. Talk to your heating engineer about installing a limescale reducer to act as a filter or use a limescale silencer to remove build-up if the problem is further advanced.
• Central heating sludge – Central heating sludge causes rust and debris to infiltrate the water. It is recommended that your heating system receives a flush every 5 years to reduce build-up of contamination and damage to your heat exchanger. A filter can also be installed to help catch debris as it passes through a filter.

What is a heat exchanger FAQs