Why your boiler is leaking water – and how to fix it

A boiler leak is a clear sign there’s a problem with your boiler. Whether your boiler is leaking water or leaking gas, you need to take action – and fast.

Here’s what could be causing your boiler leak, and how you might be able to fix it.

02 Oct 2020 | Help me with maintenance & repair

Why is my boiler leaking water?

Boilers shouldn’t leak any water. Modern heating systems are sealed and pressurised, so the water in the system can’t escape. But should you find your boiler leaking water, here’s what could be causing it:

  • Small leaks of water could mean you need to top up your boiler pressure – read how to do this in our guide.
  • The pressure relief valve could be releasing extra pressure from the system.
  • The heat exchanger – the important part of your boiler where cold water is warmed up with hot gas – could be damaged.
  • If you have a new boiler, the joints where the heating system pipes meet the boiler may have been badly sealed.
  • Or if your boiler is getting old, some parts could have stopped working.

All of these faults are typically repairable. But in some instances, you may need a new boiler.

How do I know if my boiler’s leaking water?

The first sign of a leaking boiler might be that it cuts out due to low pressure. And even after you top it up, it loses pressure again quickly.

Is your boiler losing pressure but there’s no leak?

Take a look at our boiler pressure guide to see if you can fix this problem.

Or you may notice water dripping from the boiler, or running down the wall it’s fixed to. Alternatively there may be water running from a small pipe passing through the external wall near the boiler. Sometimes a small boiler leak shows up as mould, damp or staining near the boiler. Or as warped skirting boards or counter tops nearby.

For boiler heat exchanger leak symptoms, you might see water dripping out from the bottom of the casing. There could even be a leak elsewhere in the system. Check all visible pipes and joins for leaking water, and look for staining on nearby walls, ceilings or woodwork too.

Remember: if it’s a cool day, you may notice lots of steam coming from the boiler flue. This is normal, especially if you’ve a modern condensing boiler. And the good news is, it doesn’t mean you have a boiler leak.

gas boiler broken and leaking

Help – I’ve got a boiler water leak!

If you can see that your boiler is leaking water, put a bucket, bowl, or towel underneath it to prevent water damage or staining. It’s also wise to turn off the electricity to the boiler. This will remove any risk of electric shock and reduce the chance of damage to your heating system.

If you can’t see what’s causing the water leak, check the boiler pressure gauge. This is a round dial that’s usually located on the front of your boiler.

If the arrow is pointing in the upper red zone, it may be that water is escaping from the pressure relief valve. Check first that the filling loop, key or lever isn’t stuck open and allowing water into the system. Assuming it’s not, wait for the system to cool down and release the boiler pressure by bleeding the radiators. Get more advice in our guide to boiler pressure.

If water continues to leak – or the gauge heads straight into the red after you switch the boiler back on, it’s time to arrange a boiler repair. Continued leaking is a sign that there’s an underlying issue – perhaps an internal part of the boiler has broken down. And this means it’s time to call in an expert.

fixing a gas boiler leak

How to fix a boiler leak

It’s usually best to leave boiler repairs to the experts. But if you’re competent at DIY you may be able to do a basic boiler leak repair. If water is dripping from one of the pipe joins, dry the area around the join and double-check that the water is coming from it. If so, tighten the fitting a little and see if it stops.

Can I use a boiler leak sealant or sealer?

For a very small boiler leak repair, you may be able to treat your central heating system with additives that automatically form a boiler leak seal. These can be tricky to add, though. And in some cases, using a boiler leak sealer is like applying a plaster to a wound – it provides some temporary relief, but it doesn’t fix the original problem . So if in doubt, get a heating engineer in to find the cause of the leak and fix it properly.

boiler emergency service repair man

How to avoid a boiler leak

Regular servicing should pick up on any problems and prevent any boiler leaks. But should you be unlucky to have a leak, follow the advice above to try and diagnose the problem and minimise the damage. Remember, however, that boilers are complicated and potentially dangerous. So for safety and peace of mind, it’s always best to have repairs carried out by a professional engineer.

Is my boiler leaking gas?

Boilers should never leak gas or oil. For safety, modern boilers burn gas or oil in a sealed combustion chamber. They bring in the oxygen needed to make the fuel burn, and then release the exhaust fumes safely back outside.

This sealed system keeps poisonous gases out of your home. But, very occasionally, things can go wrong.


What do I do if I have a boiler leaking gas?

If you have a boiler leaking gas, you might be able to smell this too. Other signs of a leak include black stains above or near the boiler. There may be excess condensation on your windows. Or the pilot light – the flame that indicates your boiler’s working – might be burning orange or yellow (rather than blue).

A boiler gas leak is very dangerous, especially as fumes from gases like carbon monoxide can be odourless. It’s why you should install a carbon monoxide detector in your home – and it’s law, in some instances.

If you suspect your boiler is leaking gas, immediately open all windows and doors and extinguish any naked flames – including cigarettes – then call the Gas Emergency Services free on 0800 111 999. Get more advice here.


What are boiler gas leak symptoms?

Boiler gas leak symptoms include:

  • Headache
  • Dizziness
  • Feeling sick
  • Tiredness
  • Confusion

If you suspect a leak and you’re feeling unwell, get into fresh air and seek immediate medical advice.

What do I do if I have a boiler leaking oil?

If you use heating oil and spot oil stains or pooling, it could be a sign that your boiler is leaking. The smell of oil might also be stronger than normal.

Close the tap or valve on the oil tank, and open the windows to prevent fumes building up. If it’s safe to do so, try to collect any leaking oil in a tub, or contain it by putting down sand or earth. Don’t wash it away!

Get help by calling out a heating engineer or your home insurer’s emergency number. And if anyone feels sick or dizzy, leave the house immediately.