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How much does a broken boiler cost to repair?

Our survey of Which? Trusted Trader boiler engineers reveals the boiler parts most likely to fail and what you'll need to pay to fix common boiler faults

How much does a broken boiler cost to repair?

With four out of ten boilers needing repair in the first six years of operation, it’s only a matter of time before most of us call a professional to fix the problem.

But while a broken boiler can feel like very bad news, it doesn’t always mean you need a new boiler – many faults can be fixed by replacing a faulty part.

According to independent boiler engineers from Which? Trusted Traders, the costs of the four most common repairs range from £98 to £268, on average. Not cheap, but certainly much more affordable than a brand new boiler.


If total disaster strikes and you do need a new boiler, get one from a reliable brand. We reveal the best boiler brands.


The most common gas boiler repairs

We asked 153 heating engineers from Which? Trusted Traders how much they would charge for parts and labour to fix the ten most common boiler faults.

We also asked, in their expert opinion, which parts are most likely to fail in a gas boiler.

The table below shows the four boiler parts that are most likely to develop a fault according to Trusted Trader engineers, plus the average cost of a repair. Costs will, of course, vary depending on factors such as your specific boiler and where you live, but our analysis should give you a good sense of typical costs.

Boiler part Average repair cost (parts and labour)
Diverter valve - Boiler repairs Diverter/Zone valve £187
Automatic Air Vent - Boiler faults Automatic air vent £98
Pressure relief valve - boiler repairs Pressure relief valve £101
Printed Circuit Board - boiler faults Printed circuit board £268

Read the full results of our boiler engineer survey and find out the costs for all ten of the most common boiler faults in our guide on common boiler problems.

Why do boiler parts break?

Read on to find out what these vital parts of a boiler’s anatomy do, what causes them to break, and signs your part might be faulty.

1. Diverter/Zone valves (average repair cost £187)

Zone Valve for boiler

What does a diverter valve do?

A diverter valve controls the flow of hot water, sending it either to radiators or to taps and showers.

The valve is only needed in boilers that heat hot water on demand – so you’ll find them in combination boilers, but not in system boilers or heat-only boilers.

Without the diverter valve, the combination boiler cannot prioritise where to send hot water.

What causes it to break?

Because it is a moving part, the most common cause of a diverter valve fault is due to it sticking in a certain position. You should always call a qualified heat engineer to free or replace a stuck diverter valve.

How can I tell it’s faulty?

The most common signs your boilers diverter vale is faulty are:

  • Your hot water taps work, but your radiators don’t: the valve could be stuck in a position that only sends hot water to your taps.
  • You only get lukewarm water from your hot tap: the valve could be stuck in a position that is sending some hot water to the radiators instead of just your taps.
  • You only get hot water when the central heating is also on: this problem is often revealed in summer, when you don’t need your central heating on. This could be due to the valve not being able to divert hot water properly.

2. Automatic air vent (average repair cost £98)

automatic-air-vent-boiler

What does an automatic air vent do?

An automatic air vent lets air out that builds up in the heating system.

Trapped air can lead to a variety of problems including noisy radiators and rusting on the inside of the radiator. As such, an automatic air vent can significantly increase the life of your heating system, and reduce maintenance costs.

What causes it to break?

Like all mechanical parts, the automatic air vent can be subject to sticking. In this case it is often the float – the piece that moves to seal the valve when water gets in – that sticks. A qualified heat engineer is best suited to diagnose and fix a faulty air vent.

How can I tell it’s faulty?

If the automatic air valve isn’t functioning properly then air can build up in your system. This can show in a few ways:

  • Your radiators are cold at the top trapped air means the hot water cannot flow through the whole radiator – giving it a cold top and hot bottom.
  • Your air vent has water leaking out of it if the valve isn’t functioning properly, then when it fills with water the valve doesn’t close and the water escapes.

3. Pressure relief valve (average repair cost £101)

pressure-relief-valve-boiler

What does a pressure relief valve do?

The pressure relief valve is an important safety component in a boiler. When the pressure in a heating system becomes too high, the valve opens allowing water or steam to pass through, and drop the pressure back to safe levels.

Without a pressure relief valve, your boiler would have to shut down as a safety precaution.

What causes it to break?

The main cause of pressure relief valves becoming faulty is from damage to the seal around the valve or by becoming blocked by debris such as rust.

If this happens you should have a qualified engineer clean or replace the valve.

How can I tell it’s faulty?

If the pressure relief valve is faulty you can often tell by the following:

  • Your system is losing pressure whilst other faults can also cause this, a broken relief valve will result in your heating system repeatedly losing pressure.
  • Your pressure relief valve is leaking if the valve isn’t functioning properly, then it may release water even when the heating system’s at normal pressure. If the valve, or the copper exit pipe (a small pipe that exits your property, usually close the boiler) is dripping then contact a heating engineer.

4. Printed circuit board (average repair cost £268)printed-circuit-board-boiler

What does a printed circuit board do?

A boilers circuit board is the ‘brain’ of the system. It monitors pressure, temperature and water currents within the heating system, making all the components of your system run together smoothly.

Without it, your boiler cannot operate.

What causes it to break?

Aside from general wear and tear, leaking water from the boiler could kill the circuit board.

Another common cause of a circuit board failure is if the soldering that connects the electronic components to the circuit board breaks off or gets damaged.

As the printed circuit board is located inside the boiler’s casing, it is essential that you have a gas safe registered engineer diagnose and fix it – removing the casing yourself is dangerous and invalidates the boilers warranty.

How can I tell it’s faulty?

Whilst the same symptom can be caused by different boiler parts failing, some that can be caused by a faulty circuit board are:

  • Your display panel won’t turn on if your boiler has an electronic display on it that’s not turning on, it could be the printed circuit board is faulty.
  • Your boiler operates intermittently as it is the ‘brain’ of the boiler a broken circuit board can cause a malfunction. This could manifest in the boiler cycling on and off for no reason.
  • Your boiler has a burning smell coming from it if there are burning smells coming from your boiler – in combination with sporadic operation or the display not turning on – then it could mean parts of the circuit board have burnt out.

How do I stop my boiler breaking?

Unfortunately, a boiler can’t be guaranteed to never fail. However, you can significantly reduce the chances of a boiler developing a serious fault by getting your boiler serviced every 12 months.

As part of your boiler service, the engineer should diagnose any issues and, if necessary, clean boiler parts.

Read our guide on what to expect from a boiler service to give you confidence that your boiler is in the best possible condition.


Does your heating system need a service? Visit Which? Trusted Traders to find an engineer that’s been vetted and approved by Which?. 


Can I fix my own boiler?

Adjusting settings on a boiler

You should never attempt any repairs that require the boiler casing to be removed. This is dangerous and invalidates any warranty. Such repairs should always be carried out by a heating engineer on the Gas Safe Register scheme (by law, anyone who works on boilers must be registered).

However some issues, such as a frozen condensate pipe or loss in boiler pressure, may have simple solutions that don’t involve work on the boiler itself and can be handled by a DIYer.

If you experience one of these issues, take a look at our common boiler problems guide to learn more about straightforward solutions to try yourself before you call out a heating engineer.

If you are looking into purchasing a new boiler, take a look at our guide on what you need to know before you buy a new boiler.


Our research: In May and June 2020, we asked 7,500 people about their experiences of boiler repairs. Four in ten said their boiler had needed a repair in the first six years of operation. In May and June 2020, we asked 153 boiler engineers from Which? Trusted Traders about the most common boiler faults and how much they cost to repair.

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