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Quick answer
The short copper pipe that pokes through the wall near your boiler, often turned back towards the wall, is the pressure relief valve discharge pipe. If it is dripping, the relief valve has opened because system pressure reached its limit (usually 3 bar), or the valve has opened in the past and is no longer sealing. The root cause is almost always high system pressure from overfilling, a passing filling loop or a failed expansion vessel. The valve itself can be replaced, but fixing the pressure problem comes first.
Key takeaways
- The pressure relief valve (PRV) is a safety device that releases water when pressure gets too high.
- A dripping discharge pipe is a symptom; the cause is usually high pressure in the heating system.
- Once a PRV has lifted, debris can stop it resealing, so it may keep dripping even at normal pressure.
- Condensate pipes are plastic and carry mildly acidic water; the PRV pipe is copper and carries system water. Do not confuse the two.
- Replacing a PRV is a job for an engineer, and they should also check the expansion vessel.
Which pipe is which
Most condensing boilers have two small pipes leaving them. The condensate pipe is usually white or grey plastic, runs to a drain or soakaway, and carries condensation from the flue gases. The PRV discharge pipe is usually 15mm copper and exits the wall, often with its end turned back so any hot water discharges safely towards the wall. If the copper pipe is dripping, it is the relief valve.
Why the relief valve opens
The valve is set to open at a fixed pressure, typically 3 bar on domestic sealed systems. Pressure reaches that level when:
- The system has been overfilled during topping up.
- A filling loop valve has been left slightly open or is passing water.
- The expansion vessel has lost its air charge, so pressure spikes as the water heats.
We explain each of these in boiler pressure too high.
Why it keeps dripping
When a PRV lifts, it can carry tiny particles of debris onto its seat. After that, even at normal pressure, it may not seal fully and will drip. So a dripping PRV has two possible states: pressure is still too high, or pressure is now normal but the valve is damaged. Check the pressure gauge with the system cold. If it reads between 1 and 1.5 bar and the pipe still drips, the valve probably needs replacing.
What an engineer will do
- Check cold and hot system pressure.
- Test the expansion vessel charge and recharge it if needed.
- Check the filling loop for passing.
- Replace the pressure relief valve if it is not sealing.
- Refill and repressurise the system, then check for leaks.
On some combis, the relief valve and expansion vessel are inside the boiler casing, so this work must be done by a Gas Safe engineer.
What not to do
Do not cap, block or bend the discharge pipe to stop the drip. The relief valve is a safety device and its outlet must stay clear. A blocked relief outlet on a system with high pressure removes one of the boiler’s protections.
If the valve keeps discharging after the pressure is corrected, book a boiler repair so the cause is found rather than just the symptom.
Book with Gasmaster
Copper pipe dripping outside? Gasmaster Plumbing & Heating, Gas Safe registered (220932), repairs relief valves, expansion vessels and boilers across Stoke-on-Trent, Newcastle-under-Lyme and Biddulph. See our boiler repair service.
Frequently asked questions
Is a dripping pressure relief valve dangerous?
The drip itself is not dangerous; it shows the safety valve has done its job. Leaving the underlying high pressure unfixed is the concern.
Will my boiler lose pressure because of the drip?
Yes, over time. The system will drop pressure, and you may find yourself topping up, which can restart the cycle.
Can I replace the relief valve myself?
On most combis it is inside the appliance and should be replaced by a Gas Safe engineer.
Why is water coming from the overflow pipe instead?
A plastic overflow pipe from a toilet cistern or loft tank is a different problem. See overflow pipe dripping outside.
Sources
- Gas Safe Register (gassaferegister.co.uk)
- Building Regulations Approved Document G, hot water safety (gov.uk)
- Manufacturer installation manuals for sealed-system boilers
Related services
Abdul runs Gasmaster Plumbing & Heating and has worked on boilers, heating and gas appliances across Stoke-on-Trent since 2005. He is Gas Safe registered (reg. 220932), and these articles come from his day to day work in homes and businesses across Staffordshire, Cheshire and Derbyshire.
