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Boiler leaking water or gas - and how to fix it

Whether your boiler is leaking water or 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.

Why is my boiler leaking water?

Boilers shouldn’t leak any water but should you find your boiler leaking, here’s what could be causing it:

  • Small leaks of water could mean you need to top up your boiler pressure
  • The pressure relief valve could be releasing extra pressure 
  • The heat exchanger – the important part of your boiler where cold water is warmed up with hot gas – could be damaged
  • The joints where the heating system pipes meet the boiler may have been badly sealed.
  • 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.

Look at new boilers

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. If that happens there are a couple of things you can do:

  • Take a look at our boiler pressure guide to see if you can fix this problem.
  • Check the area around the boiler for any leaks. There could be water running down the wall the boiler is fixed to or 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. It can even be warped skirting boards or countertops nearby.

My boiler is leaking water - what do I do?

Here are few things you can do when your boiler is leaking:

  1. It's wise to turn off the boiler's electricity supply. This will remove any risk of electric shock and reduce the chance of damage to your heating system.
  2. 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.
    • If it's not stuck open, wait for the system to cool down and release the boiler pressure by bleeding the radiators.
    • 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. 
gas boiler broken and leaking
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 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.
  • For a very small boiler leak repair, you may be able to treat your central heating system with additives that automatically form a seal. These can be tricky to add though and only provide a temporary solution. It's best to call a heating engineer who can fix it properly.

How to avoid a boiler leak

It's best to have your boiler regularly serviced. That way a professional engineer can pick up any potential problems and make sure your boiler's in top condition. Aim to service it once a year.  

It's also worth looking at boiler cover as many insurance providers, including our partner Domestic and General, throw in boiler inspection and servicing

boiler emergency service repair man

Is my boiler leaking gas?

If your boiler's leaking gas, you may be able to smell it. 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).

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 my boiler is leaking gas?

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

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. Here are some symptoms to be aware of:

  • 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 from 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.