Common boiler problems and repair costs

Boiler not working as it should? Check for one of these common faults

The first port of call for diagnosing the problem should be your boiler's manual. It's a font of knowledge that will explain fault codes and let you know if you can fix the problem yourself. 

If you can't find your manual, most manufacturers will have an online copy available to download on their websites.

To help with diagnosis and prepare you for the likely cost of putting problems right, we've outlined the most common faults your boiler may develop, and how much you should expect to pay for common repairs.

Need a heating engineer to fix your boiler? Make sure you use a Which? Trusted Trader. All our traders have passed our rigorous vetting procedure carried out by our trading standards professionals. 

You can use our Trusted Traders search tool below to find a local trader in your area.

Common boiler faults

If you're not a trained heating professional, you should never attempt any repairs that involve removing the boiler casing. It's dangerous to do so and also invalidates any warranty you may have remaining on the boiler. 

However, two of the three most common boiler faults can be handled by a competent DIYer and don't always require the attention of a costly heating engineer. 

1. Boiler losing pressure

If your boiler loses pressure it could be one of a few things. Most commonly, it will either be a leak, you've recently bled your radiators and the system has lost pressure or your heating has been off for a very long time and lost pressure. 

You can repressurise your system yourself by opening the filling loop taps on your boiler until you achieve the desired pressure (usually around one bar). Once this is done, remember to close both taps, at which point your boiler should start to work again. 

Watch: How to repressurise your boiler

If your boiler is consistently losing pressure you should have it looked at by a trained engineer to fix the issue.

If your radiators heat inconsistently, they may need balancing. Read our how to balance your radiators guide to find out how it may reduce your heating bills

2. Frozen boiler condensate pipe

When there's a cold snap, it's remarkably common for a frozen condensate pipe (a condensate pipe is a feature of condensing boilers) to put your boiler out of action. It's not technically a fault with the boiler but if the condensate pipe freezes, the boiler will shut itself down for safety reasons. 

Before you incur a costly call-out charge from an engineer, always make sure you've checked that the condensate pipe isn't the problem. Always make sure your condensate is properly insulated with foam, as in the image above, as this should prevent it from freezing in the first place.

Learn how to defrost the pipe safely in our guide on how to thaw a frozen boiler condensate pipe

3. Water leaking from the boiler

This is usually the result of old and corroded washers. You will need a trained heating engineer to open up the boiler and replace them for you.

If it's time for a new boiler, read our best gas and oil boiler brands guide to find the most reliable brand

Dangerous boiler faults

Faulty boilers can leak carbon monoxide (CO), a colourless, odourless gas that is produced by incomplete burning of carbon-based fuels. It stops the blood carrying oxygen around the body and can be lethal. According to the NHS carbon monoxide kills around 60 people a year in England and Wales.

Owners of old-fashioned non-room sealed boilers and systems with hidden flues are at a higher risk from the dangers of carbon monoxide. Signs of a boiler CO leak can include dark, sooty staining around it, or a sudden increase in the amount of condensation on the inside of your windows.

You should get a CO detector, and if you live in Scotland you legally need one in your home.

However, don't just buy any old CO detector. Which? testing has uncovered several dangerous CO detectors. Often unbranded and from online marketplaces, our tests have found they fail to alarm even in the presence of large amounts of CO. Check our Don't Buy carbon monoxide detector reviews to know which ones to avoid.   

The best way to protect yourself from the dangers of carbon monoxide is to buy one of our best carbon monoxide detectors that will alert you if a dangerous leak occurs.  An annual service is also essential to make sure that your boiler remains safe. 

Symptoms of carbon monoxide poisoning

Symptoms of CO poisoning can include the following. If you experience these and are concerned they could be linked to CO poisoning, leave the property that has a suspected leak and seek medical help (either from your GP or by going to A&E, depending on the severity). 

• Headaches
• Breathlessness
• Nausea and/or vomiting
• Dizziness and/or drowsiness
• Chest and/or stomach pains
• Visual problems
• Loss of consciousness. 

What to do when a carbon monoxide detector goes off – steps to take to keep yourself safe

How much do common boiler repairs cost?

While frozen condensate pipes, leaking water and dropping boiler pressure are relatively common, there are lots of specific technical faults that can occur with your boiler.

When a part breaks and needs repairing, you'll need a heating engineer to buy and carry out the repair for you.

In 2023 we asked 198 Which? Trusted Trader heating engineers how much they would charge for parts and labour to fix the 10 most common boiler faults. 

Members have exclusive access to the average price our engineers quoted so Join Which?, or log into Which? today to make sure you're not being ripped off. 

Boiler part	Boiler part name	Cost to replace part (including installation)	What this boiler part does
	Diverter/zone valves		Directs flow of heated water from the boiler for heating or hot water
	Printed circuit board		The ‘brain’ – it makes things run together
	Water Pump		Moves heated water from the boiler, for either heating or hot water
	Fan for combustion		Moves air through the boiler for combustion
	Gas valve		Regulates gas flow to the burner
	Overheat thermostat		Prevents boiler overheating
	Automatic air vent		Lets air out of the boiler

Show all rows

If you need a new boiler, our expert guide on buying the best boiler will help you get the best installation

How much is a boiler service?

A boiler service costs between £70 and £110 (not including any resulting repairs). 

Worryingly, government figures show that the majority of people don't get their boilers serviced annually. 

An annual boiler service is the best way to keep your boiler in good working order, to diagnose problems early and limit costly repairs. 

Failure to get your boiler serviced annually will also, in most cases, invalidate its warranty. And could mean that your boiler is working less efficiently than it should be, costing you more in energy bills. 

Read our guide on getting the best boiler service to make sure all the important checks happen or call a Which? Trusted Trader to book a boiler service.