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Boiler Pressure Relief Valve Leaking or Dripping? What It Means
If the small copper pipe coming out of the wall near your boiler is dripping, or the boiler keeps losing pressure, the pressure relief valve is letting water out. The valve is a safety device that opens at about 3 bar, so it is usually doing its job because something else has pushed the pressure too high: an expansion vessel that has lost its air charge, a filling loop left open or letting water by, or, on a combi, a failed plate heat exchanger. Once a relief valve has opened a few times it often fails to reseal and keeps dripping even at normal pressure. The fix is to find and repair the cause, then replace the valve if it will not reseat. Emergency Plumber Chelsea repairs boiler pressure faults across SW3, SW10 and nearby postcodes on 020 3002 6667.
Updated July 2026 · Emergency Plumber Chelsea
First, make sure it really is the boiler relief valve. Several pipes can drip outside a London house. The boiler pressure relief discharge pipe is usually a narrow copper pipe, about the width of a finger, coming straight through the wall near the boiler and often turned down towards the ground. A plastic pipe of a similar size running to a drain or gully is more likely to be the condensate pipe, which carries a little water away all the time when the boiler is running and is not a fault. A wider plastic pipe higher up, from a bathroom, is usually a toilet or cistern overflow, and our guide to a dripping overflow pipe covers those. A pipe from a tundish near an unvented hot water cylinder is a separate safety discharge, and our guide to a leaking hot water cylinder deals with it. If the drip is into a small funnel near a hot water cylinder, our guide to what a tundish is covers that instead.
Check the pressure gauge. With the heating off and the system cold, most combi and system boilers should read around 1 to 1.5 bar. Put the heating on and watch it for half an hour. A rise of about half a bar as the water warms is normal. If it climbs towards 3 bar, the relief valve will open to let the excess out, and you may see the needle drop back as the pipe outside drips. That pattern tells you the valve is working properly and the cause is elsewhere. If the pipe drips all the time, even when the gauge reads well below 3 bar, the valve itself is no longer sealing.
A flat expansion vessel is the most common reason the pressure spikes. The expansion vessel is a small tank, inside most combis and next to many system boilers, with a rubber diaphragm and a charge of air on one side. As the heating water warms and expands, it pushes into the vessel and the air cushion absorbs it. Over time the air charge leaks away, the vessel fills with water, and there is nowhere for the expansion to go, so the pressure shoots up every time the heating runs and the relief valve opens. An engineer can test the vessel, recharge it with a pump, or replace it if the diaphragm has split. Our guide to boiler pressure too high explains the pattern in more detail.
A filling loop that is open or letting by is the next thing to check. The filling loop is the braided hose or built-in valve under the boiler that you use to top up the pressure. If it has been left open, or one of its valves no longer shuts fully, mains water keeps creeping into the heating system, the pressure rises even when the heating is off, and the relief valve discharges. Make sure both valves on the loop are firmly closed. If you have an external loop, a qualified person will normally disconnect it after topping up so it cannot let by. If the pressure still rises with the heating cold and the loop closed, the valve may be passing internally.
On a combi, a failed plate heat exchanger can do the same thing. The plate heat exchanger keeps the mains water that goes to your taps separate from the water in the heating circuit. If an internal plate cracks, mains water, which is at a higher pressure, can leak into the heating side and push the pressure up, particularly after you have run a hot tap. It is less common than a flat expansion vessel, but it is worth an engineer testing for if the vessel and filling loop are both fine.
Why the valve keeps dripping afterwards. Every time a relief valve opens, it flushes a little debris and scale across its seat. After a few openings it often fails to close fully and starts to weep even at normal pressure, so the boiler slowly loses water and eventually locks out on low pressure. That is why it can look as if the problem has switched from pressure too high to pressure too low. Our guide to a boiler losing pressure explains how to top up and when a slow loss means a leak somewhere else.
What you can safely do yourself. Keep the pressure between about 1 and 1.5 bar when cold, and do not overfill when you top up, because that only makes the valve open again. Close the filling loop fully after use. If the discharge pipe ends inside, for example into a cupboard or a tundish, put a container under it and keep an eye on it. Never block, cap or tape over the end of the relief pipe to stop the drip; it is there to release pressure safely, and blocking it defeats the safety device. Do not open the relief valve by hand to lower the pressure either, as it may not reseal afterwards.
Why it is worth fixing promptly. A dripping relief valve is not usually dangerous in itself, but it causes knock-on problems. Every time you top up, you add fresh, oxygen-rich water to the system, which speeds up internal corrosion and sludge. The drip can stain brickwork and, in winter, the pipe outside can drip onto a path and freeze. In Chelsea mansion blocks and converted houses, where the discharge pipe often runs a long way through cupboards and floors, a weeping joint on that pipe can go unnoticed and damage the flat below.
What the repair involves. The relief valve on most combis is inside the boiler casing, so replacing it is a job for a Gas Safe registered engineer, and it means draining the boiler's water side, fitting the new valve and refilling and bleeding the system. A good engineer will not just fit a new valve, though; they will test the expansion vessel charge, check the filling loop and, on a combi, rule out the plate heat exchanger, because a new valve on a system with a flat vessel will simply start dripping again. If the system water is dirty, a clean-out or radiator power flush may be recommended at the same time.
If your boiler overflow pipe is dripping, the pressure keeps climbing towards 3 bar, or you are topping up the boiler every few days, call Emergency Plumber Chelsea on 020 3002 6667. Our boiler repair service replaces relief valves, recharges and replaces expansion vessels and fixes filling loops across SW3, SW10, Knightsbridge, Sloane Square, South Kensington and Fulham. There is no call-out fee on jobs over 3 hours.
Quick tips
- A narrow copper pipe dripping near the boiler is usually the pressure relief discharge.
- Watch the gauge as the heating warms up: a climb towards 3 bar points to the expansion vessel.
- Pressure rising with the heating off points to the filling loop letting water by.
- Never cap or block the relief discharge pipe to stop a drip.
- Top up to about 1 to 1.5 bar cold and no higher.
FAQs
Why is my boiler pressure relief valve dripping?
Usually because the pressure has been rising to around 3 bar, most often due to an expansion vessel that has lost its air charge or a filling loop letting water in. After opening a few times, the valve itself often stops sealing and keeps dripping at normal pressure.
Is a leaking boiler pressure relief valve dangerous?
Not usually in itself, because the valve is doing its safety job. But it means the system is losing water and something is pushing the pressure too high, so it should be checked. Never block the discharge pipe.
Can I replace a boiler pressure relief valve myself?
No. On most boilers the valve is inside the casing, and the work must be done by a Gas Safe registered engineer. You can check the gauge, close the filling loop and avoid overfilling while you wait.
Why is water coming out of the pipe outside my boiler?
If it is a narrow copper pipe, it is almost certainly the pressure relief discharge. If it is a plastic pipe running to a drain, it is probably the condensate pipe, which is normal while the boiler runs. A larger plastic pipe higher up is usually a toilet or cistern overflow.
Will a new relief valve stop the drip?
Only if the valve was the whole problem. If the expansion vessel is flat or the filling loop is letting by, the new valve will start dripping again, so the cause should be fixed at the same time.
Need an engineer in Chelsea?
Call 020 3002 6667 for 24/7 emergency dispatch across SW3, SW10 and nearby postcodes.