Fix a dripping pressure relief pipe outside

Check your boiler pressure, clear the outside pipe, then know when a dripping relief valve needs a professional.

AB-01Find the pipe and work out what it isTrace the dripping pipe back from where it comes through the outside wall. A copper pipe ending a metre or so above ground, often with a visible open gap (called a tundish) partway along, is a pressure relief pipe from your boiler or hot water cylinder. A white plastic pipe dripping clear water is usually the condensate pipe, which is a different, mostly harmless issue. This guide covers the copper pressure relief pipe.

1 each · Torch

Buy:ScrewfixB&QToolstationAmazon

AB-02Check your boiler or cylinder pressure gaugeFind the pressure gauge on the front of your boiler, or the gauge on your hot water cylinder if you have one. Most systems should read between 1.0 and 1.5 bar when cold — check your boiler's manual for the exact figure, as it varies by model. If the needle sits in the red zone, the system is over-pressurised, and that's usually why the relief valve is dripping.

AB-03Bring the pressure down safelyIf pressure is too high, bleed one radiator slightly using a bleed key until the gauge drops back into the correct range, then close it firmly. Catch any drips with an old cloth or bucket. Also check the filling loop (the flexible braided pipe near the boiler) is fully closed at both ends — a loop left slightly open keeps refilling the system and pushing the pressure back up.

1 each · Radiator bleed key

Buy:ScrewfixB&QToolstationAmazon

1 each · Old cloth or towel

Buy:ScrewfixB&QToolstationAmazon

1 each · Small bucket

Buy:ScrewfixB&QToolstationAmazon

AB-04Clear the outside pipe outletCheck the open end of the pipe outside for a blockage — leaves, cobwebs, or insects can sit in the opening. In freezing weather, ice can block the outlet and force water back up, which sometimes shows as dripping higher up the pipe. Clear debris gently with a stick, and thaw ice by pouring warm (not boiling) water over the pipe rather than chipping at it, as chipping can crack the fitting.

AB-05Leave it and check againWait a few hours, or overnight, then check the pipe outside again. If the pressure was the cause, the dripping should have stopped completely. If it's still dripping steadily at the same rate as before, the fault isn't the pressure — the relief valve itself needs attention.

AB-06Act fast if it becomes a steady flow, not a dripA slow drip is a warning sign. A steady stream of water is different — it can mean the system is failing to control pressure or temperature properly. If this happens, turn off the water at the stopcock (usually under the kitchen sink — check now that it turns before you ever need it in a hurry), turn off the boiler, and call a heating engineer straight away.

AB-07Know when to call a qualified engineerIf the pipe keeps dripping once the pressure is correctly set, the pressure relief valve itself is usually worn and needs replacing. This is not a DIY repair, because the valve is a safety device. Book a Gas Safe registered engineer if the pipe runs from a gas boiler, or a G3-competent engineer if it runs from an unvented hot water cylinder.

RULES

KNOW THE RULES

Work on a gas boiler's internal parts must be done by a Gas Safe registered engineer, by law. Work on an unvented hot water cylinder's safety and pressure relief valves must be carried out by someone registered as competent under Building Regulations Part G3 — this is a legal requirement, not just good practice.

Some links may earn us a small commission at no extra cost to you.

Save this as a project

Back to all guides