Why Is My Boiler Leaking Water?

18 July 2023

Whether it’s a trickle, a puddle or a deluge, seeing water beneath your boiler can be quite distressing. One thing is certain here – something has gone wrong. 

The most important thing to remember now is that your first port of call should be a qualified Gas Safe Registered engineer. It’s far too dangerous to try to fix the boiler yourself. Inside the cover, there’s a mix of gas, electricity and water (which may be boiling). That’s no place for an amateur.

If you discover a leak, you should turn your boiler off straight away, as you don’t want water to leak onto electronics inside or outside the boiler. If turning it off doesn’t stop the leak, there could be a problem with a valve or the mains inlet pipe – the engineer should be able to diagnose it quickly. 

However, it might be worth turning off your water supply, or the mains supply to the boiler if you can find its valve.

That said, you’ll probably want to know what could have failed, if only to estimate what the cost of repair will be. It’s also useful to let the engineer know what kind of leak is occurring before they come out.

What’s inside the boiler?

If you’ve ever looked inside a boiler, you’ll probably have noticed that it’s a maze of pipes, pumps and electronics. They are pretty compact devices, perfected over decades to be efficient and reliable, and that can make it look more complicated than it is.

Domestic boilers are either combi or conventional types. If your boiler feeds both the radiators and your hot taps directly, it’s a combi boiler. With a combi boiler, you don’t have to wait for your hot water to heat up – it does it on demand, instantly.

If your boiler fills a cylinder and that’s where you get your hot water from, it’s a conventional type. If you want hot water and someone in the house has just had a bath, you’ll have to wait for the water to heat up.

Combi boilers have a mains water inlet that’s always open. It needs that to ensure you’ve always got hot water when you need it, at the required pressure. 

The boiler boils water over a gas or oil flame, and that water is circulated either around the central heating system or around the heat exchanger, which heats the fresh water from the mains. The hot water that comes out of your tap doesn’t itself pass over the flame.

With a conventional boiler, the mains water inlet is closed, as the water inside is circulated around your radiators and cylinder. The hot water you get from the taps has not passed through the boiler itself – it is heated by a coil in the cylinder, and that’s where the mains water inlet is located. The boiler itself is only connected to the mains so that the pressure in the loop can be topped up and, of course, so it can be filled in the first place.

So where is the leak coming from?

Knowing what’s inside the boiler should give you some clues as to where the leak is originating. Here are the commonest causes.

Mains water

Water comes in at a relatively high pressure, so any holes in the mains pipework in or around the boiler will cause dripping. It could be caused by corroded pipework or a seal might have perished. There could also be a problem with the inlet valve in a conventional system. Either way, it needs to be fixed. 

Circulating water

If the leaking water is hot or warm, then it’s likely to be from the system circulating around the radiators and/or cylinder. This issue will probably be accompanied by a drop in pressure in the system. If yours has fallen below 1 bar, a leak is likely to be from the water inside the system, which is continually circulated.


All boilers need a pump to move the water around the system. Pumps have quite a lot of moving parts and seals, and any of these can fail, causing a leak. 

A pump failure will usually mean that the whole pump has to be replaced, as they are difficult to repair. It’s better to have a brand new pump anyway, as it’ll have a complete set of new parts and a new motor, rather than some new and some old ones.

Expansion vessel

The expansion vessel is a device that keeps the water pressure consistent as it heats up and cools down multiple times a day. Inside the chamber there’s a rubber diaphragm that absorbs the pressure, and over time this can weaken and start to leak. 

There are also connections between the vessel and the hot water system, and these can degrade over time and start leaking. 

A new expansion vessel should fix the leak. Incidentally, if you have a conventional open-vented boiler setup (i.e. if you have a hot water cylinder and two tanks in the loft) there won’t be an expansion vessel, as the heating system feed tank regulates the pressure.

Pressure relief valve

If the pressure gets too high in the system, which can happen if there’s a blockage or if it has been overfilled, the pressure relief valve will open, and water will come out. In itself, this isn’t technically a fault – it’s a safety device that’s working properly. 

It is a sign that something else is wrong, though. If you think you might have overfilled the system, it could be this valve that’s causing the drip. 

Try bleeding a radiator to see if the leak stops. If it continues, there’s probably something wrong elsewhere, perhaps with the filling loop or with a restriction in the pipework or heat exchanger. That’s a job for a professional, so give us a call.

Always have your leak checked

The suggestions above should help you to work out what’s causing a leak in your boiler. However, problems left to their own devices will only get worse, and that means repair bills and potential damage will grow. 

For your own peace of mind and safety, we’d always recommend you call out a qualified Gas Safe engineer to check it out and properly diagnose the issue.