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Home & garden.

Updated: 25 May 2022

Five signs you need a new boiler

Boiler faults aren't always obvious. Here are some warning signs that your boiler needs replacing.
Sam Morris
Lead 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 tell-tale signs that it's time to consider replacing your boiler.

Need a new boiler? Head straight to our shortlist of the best boiler recommendations.

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 a sluggish radiator could mean that your boiler is on the way out.

But it could also mean that your radiators need bleeding, or 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.

So if your home takes too long to heat up or your water doesn’t come through as hot as it used to, it might be time for a boiler service or a full replacement.

Arrange for a heating engineer to come and fix the problem. If it's a gas boiler, make sure you look for a Gas Safe registered engineer, or an Oftec-qualified engineer if you have an oil boiler.

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 will 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 to not burn properly. Any faint smells could indicate a carbon monoxide leak. 

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

  • The boiler is burning a yellow flame instead of a crisp blue flame
  • Dark soot stains on and around the appliance
  • 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, you should evacuate your home straight away and call 0800 111999 (the emergency gas line) to get the leak inspected.

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

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 will be legally obliged to have one from February 2022 onwards.

Four in 10 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 a model you can trust.

3. Your energy bills are increasing

If your bills are increasing faster than normal, it could be down to an inefficient boiler. 

As a heating system gets older, it can lose its efficiency and start to cost more to run. An easy way to check your boiler’s efficiency is to look at the rating on its ErP energy label. It is a seven-point colour scale, starting from dark green (A-rated and most efficient) 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, with the tools to help you reduce your energy bills and heat your home more sustainably.

If you replace an old G-rated boiler with a modern A-rated boiler that has a full set of heating controls, you could save between £45 and £300 a year on your heating bill (source: Energy Savings Trust, August 2021)

If you’ve upgraded your boiler but your bills are still rising, you might want to consider switching your energy supplier. Use Which? Switch to compare energy prices and find the best energy deal.

4. Your boiler is leaking

If puddles appear around your boiler, you should take immediate action. Internal parts may be faulty or 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 will be 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 out immediately.

If you're ready to get a new boiler, read our guide on the  best boilers 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 are sounds you shouldn't be hearing. 

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 – turning it off could help to 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 local recommended heating engineer. Or you can use our free Trader search tool below.