The short answer
If your boiler is leaking, switch it off at the fused spur, turn off the water supply to it if you can reach the valve safely, and put a bowl under the drip. Where the water comes from tells you how serious it is. A drip from the small outside copper pipe is usually the pressure relief valve. A slow drip from a pipe joint is usually a worn seal. Water from the casing itself is a component failure. Most leaks cost £90 to £400 to fix. Never open the boiler case yourself.
What the law requires
First, make it safe
Do these four things before trying to work out the cause.
Switch the boiler off at the fused spur next to it, or at the consumer unit if you cannot find the spur.
Isolate the water. Find the isolation valve on the cold supply pipe under the boiler and turn it across the pipe. If water is running heavily, turn off the stop tap at the property instead.
Keep electrics dry. If water is near sockets or the consumer unit, switch off the electrics for that area and do not stand in water.
Catch the drip. Put a bowl or tray underneath and take a photo of where the water is, with the date. An engineer will want it.
If you smell gas at any point, do not touch any switches. Open windows, leave the property and call the National Gas Emergency line on 0800 111 999.
Where is the water coming from?
The position of the leak narrows the cause quickly. Wipe the area dry with a cloth, then watch where fresh water appears.
From the small copper pipe on the outside wall.
This is the pressure relief discharge pipe. Water here means the system pressure went above about 3 bar, or the relief valve is worn. The usual cause is a flat expansion vessel or overfilling. It is a common fault, and rarely an emergency.
From the white or grey plastic pipe outside.
That is the condensate pipe. A little water there is normal. A steady flow is not, and a frozen pipe can make the water back up and leak inside the boiler instead.
From a pipe joint or valve under the boiler.
A weeping compression joint, a perished washer on an isolation valve, or a worn filling loop connection. These are inexpensive fixes.
From a radiator or its valve.
A pump seal, the heat exchanger, the pressure sensor housing or an internal joint. Water coming from the body of the boiler is a repair for a Gas Safe engineer, and sometimes a sign the boiler is near the end of its life.
From the ceiling below a bathroom or kitchen.
Trace the pipework above. Do not assume it is the boiler.
What each leak usually costs to fix
Fixed price quoted after diagnosis in London.
- Pressure relief valve replacement: £130 to £230 fitted.
- Expansion vessel recharge or replacement: £90 to £330.
- Condensate pipe clear or reroute: £110 to £220.
- Pipe joint, isolation valve or filling loop: £90 to £180.
- Pump seal or pump replacement: £220 to £420.
- Heat exchanger replacement: £450 to £750 fitted.
- Leak detection under floors: £120 to £400 including repair.
If the heat exchanger is the cause and the boiler is over twelve years old, ask for a replacement quote. Our repair or replace guide shows the half-cost rule.
Leaks are priority visits across London. Tell us where the water appears and we will usually name the likely cause on the phone.
What not to do
- Do not open the boiler casing. It is illegal for anyone but a Gas Safe engineer to work on gas appliances, and water inside the case near electrics is dangerous.
- Do not keep topping up the system to keep the heating running. You are feeding the leak and, if the vessel is flat, making the relief valve dump more water.
- Do not plug or cap the relief valve discharge pipe. That removes the safety valve’s outlet.
- Do not ignore it. A small weep corrodes components and can become a large leak in a cold snap.
When a leak is an emergency
Call an engineer straight away if water is running steadily, if it is near electrics, if the boiler is in a cupboard with stored items soaking up water, or if you can smell gas. For anything slower, switch the boiler off and book the next available visit. Many leaks can be located and repaired in one visit.
Boiler leaking, quick answers
Is a leaking boiler dangerous?
The water itself is rarely dangerous, but water near electrics is. Switch off at the spur first.
Can I keep using the boiler?
Not safely. Running a boiler with a leak can lose pressure, damage the pump and worsen the leak. Switch it off until it has been checked.
Why is my boiler dripping only when the heating is on?
A pressure relief valve or flat expansion vessel. The system pressure rises as it heats, and the valve lifts.
Why is there water under my combi?
Look at the condensate pipe, the filling loop and the isolation valves first. Those are the cheap causes.
Will my home insurance cover it?
Many policies cover damage from escape of water but not the boiler repair itself. Take photos and check your policy.