Why is my boiler leaking?

Updated on 5th November 2026:

Kieran Haig

Technical Repair Engineer, British Gas

Noticing a leak coming from your boiler can be worrying, but it’s important to act fast, as something might not be working as it should be. The cause of a leaking boiler can range from a loose connection to a faulty internal component.

Learn what to do if your boiler is leaking and when to call for help.

What should I do if my boiler is leaking?

A gas leak can be a dangerous boiler safety issue. If you smell gas or suspect a gas leak, leave your home and call the National Gas Emergency line on 0800 111999.

A water leak doesn’t necessarily mean a gas leak as well, but you should still get a Gas Safe engineer to investigate it as quickly as possible. 

In the meantime:

• Don't remove the boiler casing or attempt internal repairs
• Switch off the boiler using its normal controls if it’s safe to do so
• Never and touch wet switches or wiring and keep away from water near electrical equipment
• Use a container or towels to limit water damage but only if it doesn’t mean getting close to electrical hazards

Why is my boiler leaking water?

There could be lots of reasons why it’s happening - where your boiler is leaking from and any accompanying symptoms may help a professional identify the fault. Common causes of boiler water leaks include:

High boiler pressure

Excessive pressure in a sealed heating system can cause the pressure relief valve to discharge water.

Loose or broken pipe connections

Connections around the boiler can develop leaks due to worn seals, loose fittings or deterioration.

Faulty heat exchanger

Damage or corrosion to the heat exchanger can cause water to escape from the boiler.

Leaking pump or internal seal

Worn seals or a pump fault can allow water to escape.

Corrosion or age-related wear

These can affect boiler components and pipework over time, increasing the risk of leaking

Why is my boiler leaking from the bottom?

If your boiler is leaking from the bottom, it could be coming from a leaking pipe connection, seal, pump or another internal component.

However, as water can travel along pipework or surfaces, finding it underneath your boiler doesn’t necessarily mean that’s where the leak started.

Why is my boiler leaking from the pressure relief pipe?

A pressure relief valve stops the pressure from getting too high in your heating system. If there’s too much pressure, it can cause the valve to discharge water. Check your boiler’s pressure to make sure it’s in the normal range (between 1 and 2 bar).

However, if you’re repressurising your boiler over and over again without finding the cause then it may not solve the problem. If there is repeated or continuous discharge from a pressure relief valve, it could signal a mechanical issue with your boiler, such as an expansion vessel fault or a valve fault. 

It’s best to get in touch with a professional who can investigate safely.

Why is my boiler leaking when it's turned off?

Even if the leak stops when your boiler is switched off, we still recommend getting it checked out. A Gas Safe engineer can help detect the issue and get things up and running again. 

Is a leaking boiler dangerous?

Gas leaking from your boiler can be extremely dangerous and needs immediate emergency action. Not every water leak is an immediate emergency, but still needs to be investigated. 

Ignoring a boiler leak could increase the risk of:

• Water reaching electrical components of nearby sockets
• Home damage, especially to floors and ceilings 
• Water loss and falling boiler pressure
• An underlying appliance fault that needs professional attention

Acting quickly means you can prevent water damage to your home and keep your heating system running smoothly.

Can I fix a leaking boiler myself?

A leaking boiler can be frustrating, but it’s important never to try to do any internal repairs yourself. This includes removing the casing or working on gas components. 

Attempting to fix a leaky boiler yourself can be very dangerous, but you can safely check:

• Where the water seems to be coming from
• Whether the leak is constant or intermittent
• Any error codes or unusual pressure readings that you can see without opening the appliance
• If the boiler has stopped working

These can help an engineer, but they can’t replace a professional diagnosis. 

How much does it cost to repair a leaking boiler?

Repair costs depend on what’s causing the leak, the boiler make and model, any parts needed and labour. Minor repairs tend to be cheaper than those involving more complex component replacements.

Costs can also vary depending on whether your boiler is under warranty or covered by a repair plan, such as HomeCare. If your boiler is quite old, a leak could signal more costly structural wear, in which case an engineer may advise a boiler replacement rather than a repair. 

How can I prevent my boiler from leaking?

Routine maintenance could help identify wear and reveal faults in your boiler before they develop. This includes:

• Having an annual boiler service by a Gas Safe engineer, as recommended by The Health and Safety Executive (HSE) for gas appliances, flues and pipework
• Responding promptly to unusual pressure readings or recurring faults
• Investigating unexplained drops in pressure
• Following the manufacturer’s operating and maintenance guidance

If an engineer is visiting your home, keep the area around the boiler accessible so they can inspect it safely. 

Need a new boiler?

No problem. Spread the cost of a new boiler and Gas Safe-certified installation by a Which? Trusted Trader with flexible payment options from British Gas. Relax with a warranty when you service your boiler annually.