Four in 10 households at greater risk of carbon monoxide poisoning: find out if this means you

Around four in 10 people* don't get their boiler serviced annually - increasing the risk of it developing a dangerous fault, such as a carbon monoxide leak.

This Gas Safety Week (13-19September) we're urging everyone who hasn't had their boiler serviced in the past year to do so.

Gas Safety Week is a national safety campaign co-ordinated by Gas Safe Register. If your boiler hasn't been serviced in a year, now's the time to get it done. Call a Gas Safe Registered engineer to check your boiler is in good shape for the winter and reduce the risk of problems such as a carbon monoxide leak.

An annual boiler service also means your boiler is less likely to develop a fault, according to our research. Plus it will be significantly cheaper than waiting for your boiler to break and then paying for an expensive repair.

Read on to learn how to spot signs of a carbon monoxide leak, how much you should pay for a boiler service, and how much you could save by getting one.

Which? Trusted Traders - find a gas safe engineer in your local area to service your boiler.

Do boilers emit carbon monoxide?

When there isn't enough oxygen being supplied to the burning fuel in gas appliances, including your boiler, they can produce carbon monoxide.

Boilers usually carry any waste gases out of your home via the boiler's flue. But if your flue is blocked, or there's a leak in your boiler, then the carbon monoxide has nowhere to go but into your home.

Carbon monoxide is poisonous. It can make you seriously ill or even kill you.

It's also odourless, colourless and tasteless - so you can't rely on your senses to detect a leak.

When you get your annual boiler service, your engineer will check that your flue is properly maintained and the components are working as they should. This dramatically reduces the risk of your boiler leaking carbon monoxide.

Even if you get an annual boiler service, a good carbon monoxide detector is essential - but two in five models we've reviewed don't work as they should. Find out which carbon monoxide detectors failed and passed our tests

Signs of a carbon monoxide leak

Signs that one of your gas appliances (including your boiler) has a carbon monoxide leak include:

• dark, sooty staining on or around the appliance
• increased condensation on the inside of your windows
• the flame struggling to stay alight
• the flame being floppy yellow or orange, rather than a crisp blue flame. You shouldn't, under any circumstances, open your boiler to check its flame, so look at your gas hob's flame next time you're cooking.

Symptoms of carbon monoxide poisoning

Headaches, dizziness, nausea, breathlessness, collapsing and losing consciousness are common symptoms of carbon monoxide poisoning.

Find out more about signs of carbon monoxide poisoning and what to do if you suspect a carbon monoxide leak in our expert guides

Gas Safety Week

Gas Safety Week is an annual event to raise awareness of gas safety and the importance of taking care of your gas appliances. It's co-ordinated by Gas Safe Register, the official list of gas engineers who are legally allowed to work on gas.

Badly fitted and poorly serviced boilers can cause gas leaks and carbon monoxide poisoning. By taking care of your gas appliances, you'll keep your home and loved ones safe.

Follow the Gas Safe Register's recommendations:

• Check your gas appliances every year This includes getting them serviced regularly by a Gas Safe registered engineer. If you're renting, make sure your landlord arranges this. Set a reminder so you don't forget at StayGasSafe.co.uk.
• Check your engineer is qualified for the type of gas work that needs doing Not all Gas Safe Registered engineers are qualified to carry out all types of gas work. You can find this information on the back of their Gas Safe ID card and on the Gas Safe Register website.
• Install an audible carbon monoxide alarm Make sure your friends and relatives have one too. Check they're marked EN50291 and display the British Standards' Kitemark (pictured below). To find the best models, see our Best Buy carbon monoxide detectors.

Learn about other ways to avoid a carbon monoxide build-up

How an annual boiler service will save you money

Getting a service once a year is the best way to reduce the odds of needing a costly repair.

We asked 8,001 boiler owners* how often they get their boiler serviced, and whether it had ever developed a fault. The results were clear - your boiler is far less likely to develop a fault if you have an annual service.

• Only a third boiler owners that get an annual service have had to repair their boiler.
• This rises more than half owners who service their boiler every one to five years.
• Around two thirds who only service their boiler every three to five years need a repair.

A service isn't a guarantee that your boiler will never break, but the odds are in your favour if you get it serviced every year.

Find out how to get the best boiler service

The cost of a service vs the cost of one-off boiler repairs

Getting a service ensures your boiler will be in top working order. Unless you're paying for boiler cover that includes an annual service, you'll need to pay for it. But it will still be cheaper to pay for an annual service than to pick up the cost of fixing a broken boiler.

We asked 153 Which? Trusted Traders how much they charge for a one-off service, and the average price was £86.

Compare this with the cost of replacing the boiler parts that most often fail and, depending on the part, you'd typically save anywhere from £30 up to £219 by opting for the service.

The table below shows the four boiler parts that are most likely to develop a fault according to our Trusted Trader engineers, plus the average cost of a repair.

The cost will 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 (part and labour)
	Pressure-relief valve	£123
	Automatic air vent	£115
	Diverter / zone valve	£218
	Printed circuit board	£305

Find out the costs for all 10 of the most common boiler faults in our guide to common boiler problems

* May-June 2021 Which? online boiler survey of 8,001 members of the public who've bought a new boiler in the past six years