Boiler leaking water? What to do and why it happens

There isn't much you can safely fix yourself with a leaking boiler. But there are a few things you should do straight away. Most leaks then need a boiler repair from a Gas Safe engineer. This guide is part of our common boiler problems series.

What should I do if my boiler is leaking?

If your boiler is leaking, catch the water, turn the boiler off and keep water away from the electrics.Never touch a switch, plug or socket that is wet. Then turn off the water to the boiler if it's safe and easy to do.

1. Put a bowl or a towel under the drip. This protects the floor and shows how fast it's leaking.
2. Turn the boiler off at its controls.
3. If water is near the boiler's electrics, switch off its fused spur. This is the switched plate on the wall that the boiler is wired into. Only do this if the switch and your hands are dry.
4. If the switch is wet, leave it alone. Turn the power off at the fuse box instead, if you can reach it safely.
5. Turn off the water if it's easy. Some boilers have small valves on the pipes underneath. If water is pouring out, turn off the stopcock, the main water tap for your home.
6. Take a photo of where the water is coming from, and note the boiler make and model.

Most boilers run on a sealed system. This means the water in your radiators isn't topped up by the mains. So the stopcock won't stop a leak from the heating side. But that kind of leak slows down as the pressure drops.

If a pipe has burst away from the boiler, or water is coming through a ceiling, see our emergency plumber page.

Is a boiler leak a gas leak?

No. A boiler leaking water isn't leaking gas. A water leak won't make the air in your home unsafe by itself. But a smell of gas, or signs of carbon monoxide, are an emergency. Follow the safety box above and call 0800 111 999.

Leaking water doesn't give off carbon monoxide (CO), a poison gas you can't see or smell. But water inside the case can rust parts over time. A damaged boiler may not burn gas safely. So don't leave a leak running for weeks. A CO alarm near the boiler is a good idea in every home with gas. We supply and fit them.

Where is my boiler leaking from?

Where the water is coming from points to the cause. Most leaks are from a joint or valve under the boiler, the pump, the pressure relief pipe outside or the condensate pipe. A leak from inside the case often means a worn seal or a damaged heat exchanger.

Look with a torch, but don't open the boiler. Old leaks often leave clues. Look for white crusty marks, green marks on copper pipes, or brown stains on the case.

Each of these is explained below.

Why is my boiler leaking from underneath?

A boiler leaking from underneath usually has a loose joint, a worn valve or a worn seal inside. Water can run down from inside the case and drip from the bottom, so it may not start where you see it.

• Pipe joints and valves. Several pipes join the bottom of the boiler. Joints can work loose as the pipes heat up and cool down. Valves can wear out.
• The pump seal. The pump pushes hot water round your radiators. On most combi and system boilers, it sits inside the case. Its seal can wear and drip.
• Other seals inside. Rubber seals on the parts inside the boiler harden with age. Then they start to weep.

Brown or black water has come from the heating side. It's coloured by rust and sludge. Clear water may be from the heating side or from the cold water pipe that feeds your taps.

Why is the copper pipe outside my boiler dripping?

The copper pipe outside drains the pressure relief valve. This is a safety valve that lets water out when the pressure gets too high. A drip from that pipe usually means the pressure is rising too high.The valve can also wear and leak on its own.

Check the pressure gauge on the boiler. If it reads close to 3 bar, the pressure is too high. A filling loop valve left open is a common cause. So is a worn expansion vessel, a small tank that soaks up pressure as the water heats. Our guide to a boiler losing pressure covers pressure that's too high as well.

Why is my condensate pipe leaking?

The condensate pipe is a plastic pipe that carries waste water from the boiler to a drain. It can leak at a loose joint or a crack, or back up when it's blocked or frozen. A little water from its open end outside is normal.

In freezing weather, ice can block it. Then the boiler often stops and may gurgle. See our guide to a frozen condensate pipe to thaw it safely. A cracked pipe or a joint that keeps leaking needs fixing.

Does a leaking boiler lose pressure?

Usually, yes. A leak from the heating side lets water out, so the pressure gauge drops. Once it falls too low, most boilers stop working. Topping up the pressure brings it back for a while, but the leak will keep letting it out.

If the boiler is leaking but the gauge isn't dropping, the water may be from somewhere else. It may be the cold water pipe on a combi, or the condensate pipe. Those leaks don't change the pressure. They still need fixing.

Can I fix a leaking boiler myself?

Not inside the boiler. Only a Gas Safe registered engineer should take the cover off a gas boiler. By law, paid gas work must be done by someone on the Gas Safe Register.

You can do the safe things from the outside. Catch the water, turn the boiler off, check the pressure gauge and make sure the filling loop valves are closed. Don't tighten joints inside the case, and don't use sealant on a boiler.

When to call a Gas Safe engineer

Call a Gas Safe engineer for any leak from the boiler itself, or one that keeps coming back. Leaks get worse over time, and water can damage the electrics and parts inside.

• water is dripping from the boiler case or the pipes underneath
• the copper pipe outside keeps dripping
• the pressure keeps dropping and you need to top it up often
• you see brown water, rust marks or stains on the case
• water has reached the boiler's electrics

The engineer first runs a thorough check of the boiler. That finds where the water is coming from and the parts needed. See our boiler repair page for the faults we fix.

A badly rusted heat exchanger on an older boiler can cost a lot to replace. If so, we'll tell you honestly if a new boiler makes more sense.

A yearly boiler service helps prevent leaks. The engineer checks the seals, valves and joints, and can spot wear before it drips.

Other boiler guides

• Boiler losing pressure: what the gauge should read and how to top it up.
• Frozen condensate pipe: how to thaw it safely.
• Heating works but no hot water: checks to try, and what a diverter valve does.
• Cold radiators: what cold at the top or bottom means.
• Boiler won't turn on: the checks to try first, and what a pilot light is.
• Hot water but no heating: thermostat, timer and valve checks.
• All common boiler problems.

Boiler still leaking? Book a boiler repair and we'll find the cause.