Faulty Boiler Heat Exchanger – Diagnosis, Fix & Costs

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This guide will run through the common causes of a faulty boiler heat exchanger as well as exploring the next steps to repair or replace the critical boiler part should your boiler be affected.

What is a heat exchanger?

A heat exchanger is an internal part of the boiler that enables the transfer of heat from the burning of gas to the cold water, warming it up. When a heat exchanger becomes faulty, the water within the system doesn’t heat up sufficiently, leading to lukewarm baths or showers!

The heat exchanger itself has a coiled pipe running through it which, when the boiler is in use, is surrounded by cold water. The heat then passes through the coiled pipe, heating the water whilst also keeping the water and fuel separate. Heat exchangers exist in an array of other appliances such as air conditioning units, however, they would operate in the opposite way.

How does a heat exchanger work?

The heating source, for example, the gas, burns and warms the heat exchanger, which in turn, through the coiled pipe as mentioned above, heats the water.

The water is then circulated around the heating system into the radiators, heating the property. Once the water loses heat it is returned back to the boiler for the process to start again.

Common heat exchanger faults

Like most boiler internal parts, the heat exchanger can be subject to wear and tear over time, including limescale and debris build-up.

Limescale build-up

Limescale can be a large attributor to faulty parts within boilers, especially if the property is located within a hard water area. Limescale is a chalky pale substance that builds up over time causing blockages and additional wear and tear on the internal parts of the boiler, and the pipework within the heating system.

Limescale builds up gradually over time and therefore the changes to the heating system are subtle at first, however, there are some signs that the boiler is struggling with limescale such as a whistling noise caused by overheating, which creates steam inside the boiler.

The noise produced from the steam and bubbling is sometimes known as kettling. Another more obvious sign is a leak that can occur from either the heat exchanger itself or by the cracked seals. Any signs of leaks should be dealt with immediately by turning off the water supply and power and calling a Gas Safety registered engineer.

Related boiler problems:

Limescale can be prevented within a heating system by fitting a filter or inhibitor and the levels of limescale inside the boiler can also be monitored by a heating engineer during a boiler’s annual service.

If limescale has built up within the system, the damage cannot be reversed, and therefore it is likely that individual parts will need to be replaced. Heat exchanger replacements can be expensive and therefore, depending on the age of the boiler, a replacement unit may be more cost-effective and therefore all options should be discussed with the engineer.

Central Heating Sludge

In addition to limescale, other substances can also cause blockages and wear and tear within a heating system such as rust and dirt. These substances differ in colour from limescale and are often thick and brown sludge which reduces the flow of water within the system causing inconsistencies with the heat, corrosion to pipework, or in some cases, blocking the pipes.

The signs that a heating system is suffering from debris are similar to limescale in the sense that the kettling noise could be a symptom. In addition, cold parts of the heating system or radiators can be a sign as well as dirty radiator water when bleeding the radiators.

If sludge is causing performance issues within a heating system, one recommendation is for a heating engineer to perform a deep clean of the system by undertaking a system flush. There are different flushing options available from using the force of power or chemicals, however, the most suitable type will depend on the age and condition of the system’s pipework.

For maintenance purposes, it is often recommended that a system flush be performed every five years by a qualified Gas Safe Engineer.

Can you repair a broken heat exchanger?

Often repairs to a boiler’s heat exchanger are not economically viable due to the amount of labour needed from a Gas Safe engineer, and therefore it is often recommended that a replacement part is sourced.

Should a new heat exchanger be required, the first step would be to check if any warranty is in place covering the part. However, if limescale is the cause of the failure of the heat exchanger, often the warranty would not cover this.

If the warranty does not cover the part or has expired, the costs of the replacements would be to be borne by the homeowner or landlord.

Costs of replacing a boiler heat exchanger

Unfortunately, heat exchangers are one of the most expensive parts within a boiler, often costing between £400-£600 for a replacement, and therefore, depending on the age of the boiler, it is usually recommended to replace the whole boiler unit rather than replace a heat exchanger part.

Related reading: 


There is a range of causes of why a boiler’s heat exchanger would become faulty. Unfortunately, due to the costs involved with replacing the part, in some circumstances, a new boiler would be deemed more beneficial as other benefits can be gained such as energy savings, increased technology, and new warranties, covering the new boiler which can bring peace of mind.

Boiler brand faults: 

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