Central heating boiler or radiator leaking? What to do immediately

Do you smell gas? Then do not touch the boiler, do not use light switches or open flames, open windows and doors, leave the property and call the national gas and electricity fault number: 0800-9009.

What to do immediately?

1. Safety first. If water comes near a socket or the electronics of the boiler, pull the plug of the boiler from the socket or switch off the relevant circuit in the meter cupboard. Do not touch plugs with wet hands.
2. Catch the water. Place a bowl or bucket under the leak and lay down towels.
3. See where the water is coming from. Dry the bottom of the boiler and the pipes and see where it becomes wet again. Take a photo.
4. Read the pressure on the pressure gauge or display. High pressure often indicates the pressure relief valve, rapidly falling pressure indicates a leak.
5. If a radiator is leaking: isolate the radiator. Close the thermostatic valve or radiator valve and also the lockshield valve on the other side. Count how many turns you make, so it can be set correctly again later.
6. Do not open the boiler and do not try to tighten couplings inside the boiler.

Boiler leaking: the most common causes

The overflow valve

Under the boiler is a valve with a drain pipe that releases water when the pressure becomes too high. If it drips, the pressure is too high (for example from topping up too much or a faulty expansion vessel), or the valve itself no longer closes properly. The valve is a safety device: never close it.

The condensate drain

A condensing boiler produces condensate and drains it via a small hose or pipe to the drain. If this comes loose, is blocked or the siphon in the boiler is full of dirt, the condensate leaks from underneath the boiler. This water is slightly acidic; wipe it up and don't leave it on the floor.

A leaking coupling or tap

The connections under the boiler (central heating flow and return, cold and hot water, the filling tap) can start leaking due to wear or because a seal has dried out. The water then drips from a coupling.

A leak in the boiler

If the water comes from inside the boiler, it could be the pump, an internal pipe or the heat exchanger. This is always a job for a technician. With an older boiler, it's a moment to consider whether repair is still sensible; see replace or repair central heating boiler?

The hot water side

If it leaks mainly when you draw hot water, the leak is often on the hot water side of a combi boiler or at a connection on the hot water pipe.

Radiator leaking: the most common causes

• The bleed nipple. Not fully tightened after bleeding, or the seal has worn out.
• The radiator valve or thermostatic valve. Water along the spindle under the knob indicates a worn washer; water at the nut (the union) between valve and radiator indicates a leaking coupling.
• The lockshield valve at the other bottom of the radiator, for the same reasons.
• The radiator itself. An old radiator can rust through from the inside. You will then see a rust spot or a wet point on the panel, often at the bottom. Such a radiator must be replaced.
• The pipe to the radiator, in the floor or wall. If you see water but no leak on the radiator itself, the leak may be in the pipe. Then leak detection is the fastest way to find the location.

What you'd better not do

• Closing or taping shut a dripping overflow valve.
• Tightening a coupling with too much force. Older couplings crack or the pipe turns with it.
• Using leak-stop products in the system without advice. They can block the boiler and small channels in the system.
• Continuing to top up without looking for the cause.