Five signs you need a new boiler

Boilers are one of the most intensively used household appliances, but faults can go unnoticed unless the heating suddenly stops working or your water doesn’t warm up.

A boiler breakdown can be frustrating and costly, especially in the middle of winter. But certain failures can also be a danger to you and your family if not spotted in time.

Read on to find out some of the telltale signs that it's time to consider replacing your boiler.

Need a new boiler? Head straight to our reviews of the best gas and oil boiler brands.

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1. Radiators take longer to heat up

Do your radiators stay tepid or take hours to get hot? Most modern boilers can reach their maximum temperature quickly, so sluggish central heating could mean your boiler is on the way out.

But don't panic straight away. It may be that your radiators need bleeding – a relatively simple job – or that the central-heating system is filled with sludge. The latter causes blockages and cold spots on your radiators. Dirty water circulating in the system could result in long-term problems with your boiler.

All this results in your heating bills being as high as if your boiler is on full blast – but you'll only end up with a lukewarm home.

First, try our guide on how to bleed your radiators.

If your home still takes too long to heat up or your water doesn’t come through as hot as it used to, arrange for a heating engineer to take a look. They might need to flush out your central heating system or find a boiler fault. 

If it's a gas boiler, make sure you look for a Gas Safe registered engineer. If you have an oil boiler, look for an Oftec-qualified engineer.

As well as cleaning out your radiators, read our other top tips for reducing your heating bills.

2. The boiler is giving off a bad smell

A healthy boiler should never emit any odour. If it does, it needs to be investigated by a professional right away.

Carbon monoxide can be extremely dangerous and potentially fatal to you and your family. Although carbon monoxide itself is odourless, a smell could be created because the leak is causing the boiler not to burn properly. Any faint smells could indicate a carbon monoxide leak. 

Other carbon monoxide warning signs that shouldn’t be ignored include:

• the boiler burning a yellow flame instead of a crisp blue flame
• dark soot stains on and around the appliance
• a pilot light that frequently blows out
• increased condensation inside windows.

Symptoms of carbon monoxide poisoning include headaches, shortness of breath, nausea and unconsciousness.

If you notice any of these warning signs, evacuate your home straight away and call 0800 111999 (the emergency gas line) to get the leak inspected.

You and anyone else who has been in the property should be checked by a doctor. If you have a pet, take it to the vet.

The best way to ensure any carbon monoxide is detected as soon as possible is by installing a CO alarm in your home. You should have one – and if you live in Scotland you were legally obliged to have one in your home from February 2022 onwards.

A worryingly high number of carbon monoxide detectors we tested were so unreliable that we labelled them Don’t Buys. Look at our round-up of the best carbon monoxide detectors to get one you can trust.

3. Your energy bills are increasing

Gas bills are high for everyone. But if you find your gas meter is registering a higher gas usage than you'd expect, or that you've had in previous winters, it could be down to an inefficient boiler. 

As a heating system gets older, it can lose efficiency and start to cost more to run. An easy way to see how efficient your boiler should be is to look at the rating on its ErP energy label. This is a seven-point colour scale, starting from dark green (A-rated and most efficient) and going down to red (G-rated and least efficient).

In 2018, the government introduced the 'Boiler Plus' legislation, which requires all newly installed boilers to have a minimum ErP efficiency of 92%, and to have time and temperature controls fitted. So you can rest assured that a new boiler will be an upgrade in efficiency, although installation and usage will play a role. Regular servicing is key to maintaining ongoing performance.

If you live in England, Scotland or Wales and replace an old G-rated boiler with a modern A-rated boiler that has a programmer, room thermostat and thermostatic radiator controls, you could save up to £430 a year on your heating bill, depending on the type of property you live in  (source: Energy Savings Trust, July 2024). In Northern Ireland you could save up to £640 (source: Energy Savings Trust, October 2023).

That said, we wouldn't recommend replacing a boiler that's still functional. If you're worried yours isn't operating efficiently, ask a heating engineer to run a service on it and see whether there are any improvements that can be made. 

You probably don't need boiler cover, but to avoid being overcharged if you do get it, read our guide on how to choose the best boiler cover.

4. Your boiler is leaking

If puddles appear around your boiler, take immediate action. Internal parts may be faulty or could have started to disintegrate.

A leak can lead to further problems, such as rust and corrosion, as well as structural damage to your home. It will also mean that your heating is inefficient – think of the wasted hot water.

Avoid paying more for your heating and water, as well as preventing damage to your home, and call your engineer immediately.

If you're ready to get a new boiler, read our guide to buying the best boiler to find the right model and size for your home.

5. Your boiler is making funny noises

Your boiler will make a noise when firing up, but this should be no more than a low and constant hum that most of us won’t even notice. But banging, clunking and whirring sounds are cause for concern. 

These could indicate a number of issues, from a broken valve to heat-pump problems. Or, it could just be down to general wear and tear.

Always switch the boiler off first to help stop any further damage, and call out an engineer. The boiler issue could be minor and easily fixed, but it’s important to make sure the appliance is safe for use, too.

You can use Which? Trusted Traders to find a recommended heating engineer near you. Or use our free Trader search tool.