Repair or replace? Five signs you might 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. We'd always recommend repairing your boiler where you can, and keeping it going as long as possible, as a total replacement is expensive and should be a last resort. 

But there are a few signs that your boiler could need extra attention - and you'll need to decide whether to repair or replace. 

If you are thinking ahead to a boiler replacement, our guide to planning for your boiler's replacement explains the options available. 

Read on to find out some of the telltale signs that you might need to consider replacing your boiler.

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

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 just be that your radiators need bleeding – a relatively simple job – or that your central heating system needs to be flushed out. A build-up of sludge in pipes can cause blockages and cold spots on your radiators. Dirty water circulating in the system could result in long-term problems with your boiler.

First, try our guide on how to bleed a radiator.

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 they may identify a bigger 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 111 999 (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 gas bills are increasing unexpectedly

Gas bills are high for everyone. But if, when you check your statements, you find that your gas meter is burning through more kWh than you'd expect compared to previous winters, it could be down to your boiler losing efficiency. 

As a heating system gets older, it can lose efficiency and start to cost more to run. We wouldn't recommend replacing a boiler that's still functional, but if you're worried yours isn't operating as efficiently as it should, ask a heating engineer to run a service on it and see whether there are any improvements that can be made.

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 'Boiler Plus' legislation, which requires all boilers installed since then to have a minimum ErP efficiency of 92%, and to have time and temperature controls fitted. 

So when it does come to buying a new boiler, you'll almost certainly benefit from an upgrade in efficiency, although correct installation and proper usage also play a role in keeping your bills down. Regular servicing is also key to maintaining ongoing performance.

 Our free Home Energy Planning tool can help you build a personalised plan to make your home more energy efficient, and help you save money.

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. 

Avoid paying more for your heating and water, as well as preventing damage to your home, and call a heating engineer immediately to diagnose the problem. It may well be that they can do a simple fix to replace the faulty part, so ask about your options.

5. Your boiler is making funny noises

Gas boilers do make a noise when firing up, but while they're working they shouldn't make more noise than a low and constant hum that most of us won’t even notice. Banging, clunking and whirring sounds are all 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 fault 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.