Why is water running out of a pipe on my outside wall?

Five different pipes discharge water onto a UK outside wall, each meaning something different. How to identify which one is running, from its height, size and temperature.

Difficulty: beginner
Time: 20 min
Read: 5 min
Safety: warning

Short answer

Identify the pipe before anything else. High up under the eaves and cold means a loft cistern overflowing. Low down near the boiler and warm means the heating system's pressure relief valve. From a cylinder discharge and hot means an unvented safety device operating. A small pipe from a toilet or tank means a failed float valve. Never block any of them.

A pipe running water down an outside wall is always a message. Every one of these pipes exists so that a failure announces itself visibly instead of silently filling a ceiling or over-pressurising a cylinder. The mistake people make is treating it as a leak to be stopped rather than a symptom to be read.

Safety

Several of the pipes that discharge outside a house are safety devices telling you something has failed, and one of them — the discharge from an unvented cylinder — can run scalding hot. Do not plug, cap or divert any discharge pipe. If the water is hot, keep away from it and treat the situation as urgent.

Step by step

Note where the pipe is and how big it is.High under the eaves and around twenty-two millimetres: an overflow or warning pipe from a loft cistern. Low down near the boiler and around fifteen millimetres: the pressure relief valve discharge from a sealed heating system. A larger metal pipe from an airing cupboard: the discharge from an unvented cylinder's tundish.
Feel the water — carefully.Cold means a cold water cistern, a toilet cistern or a cold supply fault. Warm or hot means the heating system's relief valve or an unvented cylinder discharge. Approach a hot discharge with real caution: water from an unvented cylinder can be at or near boiling.
Note when it runs.Constantly, or only when the heating fires, or only when the hot water heats, or after somebody has used a lot of water. That timing points straight at the cause and is the most useful thing you can tell an engineer.
If it is a cistern overflow, deal with the float valve.Go into the loft and find which cistern is at fault — the big cold water one or the small heating feed and expansion one. Tying up the ball valve arm stops the flow immediately. The valve or its washer then needs replacing.
If it is the heating relief valve, check the pressure gauge.Water from a small pipe near the boiler, running while the heating is on, is the pressure relief valve doing its job. Look at the gauge: a needle that climbs steeply as the system heats and then triggers the discharge is the classic failed expansion vessel. A loop left open does the same thing.
If it is an unvented cylinder discharge, stop and call.Water from a tundish discharge is a relief valve on a mains-pressure cylinder operating. It means a failed expansion vessel, a failed pressure reducing valve, or a cylinder overheating. Switch the immersion off, turn the boiler's hot water off, and call an engineer with the unvented qualification.
Check the boiler condensate pipe as a false alarm.A small plastic pipe near the boiler that drips clear water when the boiler runs is the condensate discharge, and that is entirely normal. It should terminate at a gully or a soakaway rather than just running down the wall, but drips from it are not a fault.
Never block it.Plugging an overflow removes the only visible warning of a failure and moves the water somewhere worse — usually a ceiling. Plugging a relief valve discharge removes a safety function on a pressurised system. Fix the cause, not the symptom.

Tips

Photograph the pipe and where it comes out of the wall. Tracing it from the inside is much easier with a picture of the outside.
Green staining or moss on a wall below a pipe means it has been discharging on and off for a long time, even if it looks dry today.
In freezing weather an overflow pipe can freeze, hiding the fault until the thaw. Water appearing at a strange time of year often started weeks earlier.
Note the pressure gauge reading both cold and hot before ringing anyone about a relief valve discharge. Those two numbers are the whole diagnosis.

If it doesn't work

Cold water pouring from a pipe under the eaves

Cause: A loft cistern ball valve has failed to shut off — Fix: Tie the ball valve arm up to stop it, then replace the float valve. Establish which cistern it was — the small heating one points at a different problem.

Warm water from a small pipe near the boiler, only when heating

Cause: The sealed system's pressure relief valve is lifting — usually a failed expansion vessel or a filling loop left open — Fix: Check the loop is fully closed first, since that is free. Then read the gauge cold and hot: a steep rise on heating is the expansion vessel and needs an engineer.

Hot water from the cylinder discharge pipe

Cause: An unvented cylinder relief valve has operated — Fix: Switch off the immersion and the boiler hot water, keep away from the discharge, and call an unvented-qualified engineer the same day.

Ran once and stopped

Cause: A momentary over-pressure — often after topping up, or a one-off relief valve lift — Fix: Note the pressure. A one-off after topping up usually means the system was overfilled. If it repeats, it is a fault.

Small pipe from a toilet or basin dripping constantly

Cause: A failed float valve in a cistern with an internal overflow discharging outside — Fix: Replace the fill valve or its washer. On modern cisterns the overflow discharges into the pan instead, so an old external overflow pipe suggests older fittings.

Clear drips from a plastic pipe whenever the boiler fires

Cause: The condensate discharge — normal operation — Fix: No fault. Check it terminates properly at a gully rather than staining the wall, and insulate the external run against freezing.

Questions people ask

Can I just cap the pipe?

No, in every case. These pipes are the designed safe route for water that has to go somewhere. Capping one moves the failure into the building or removes a safety function from a pressurised vessel.

How do I know which pipe is which from outside?

Height, diameter, material and temperature. Eaves-level plastic is a cistern overflow. Low-level small-bore near the boiler is the heating relief valve. A metal pipe from an airing cupboard is an unvented discharge. When in doubt, trace it from inside.

Is it wasting a lot of water?

A running overflow can waste a great deal, and on a metered supply it shows up on the bill. It is also filling and refilling a cistern continuously, which is why the noise often accompanies it.