Water dripping from a pipe outside? Your pressure relief valve is doing its job
If you see water dripping from a small copper pipe on the outside wall near your boiler, that is the pressure relief valve. The PRV is a safety part. It opens when the pressure inside your central heating goes too high, so the boiler cannot become dangerous. Every domestic boiler in the UK has one.
A working PRV should stay dry. When it starts to weep or drip, something in the system has pushed the pressure above the safe limit, or the valve itself has worn out. Both are common. Neither is an emergency in the middle of the night, but neither should be ignored for long, because a PRV that keeps opening is a symptom, and the underlying cause usually gets worse.
What the PRV is and where it sits
The valve is inside the boiler, connected to a small copper discharge pipe that runs through the wall to the outside of the property. That outside pipe is often overlooked, because it looks like it has no purpose when everything is working. It is the only visible sign that the PRV has done something.
Manufacturers set the valve to open at around 3 bar. Normal running pressure sits between 1 and 2 bar. So a PRV that opens is telling you the pressure has climbed well above where it should be, either momentarily or continuously.
Six reasons a boiler PRV starts leaking
1. The valve seat has worn out
PRVs are spring-loaded. Over years of small pressure spikes, the internal seat weakens and the valve loses its seal. It then weeps constantly, even at normal pressure. On any boiler more than five years old, this is by far the most common cause.
Fix: replace the valve. A straightforward job for a Gas-Safe engineer, usually done in the same visit.
2. The pressure gauge is under-reading
If the gauge shows 1 bar but the real pressure is closer to 2.5, the system feels normal to you and the PRV knows better. This one traps homeowners who try to repressurise a boiler that is already at pressure, which makes the leak worse.
Fix: check the real pressure with a service gauge. Replace the built-in gauge if it is wrong.
3. The filling loop is left open, or the filling valve has failed
The filling loop is the flexible pipe that lets you top the system up from mains water. It should be closed off the moment you have finished. A tap left slightly open, or a failed filling valve that will not shut fully, keeps mains pressure feeding into the system, and the PRV vents to compensate.
Fix: close the filling loop by hand. If it will not close, or the pressure keeps climbing with it closed, the valve needs replacing.
4. The expansion vessel has lost its charge
The expansion vessel is a small pressurised tank inside the boiler, usually red. It absorbs the extra volume when heated water expands. When its air charge leaks out, there is nowhere for the expanded water to go, so the pressure jumps every time the boiler fires. The PRV opens, water dumps outside, and by the time the boiler cools, pressure drops back below 1 bar. You end up in a repressurise-and-redump cycle.
Fix: re-charge or replace the expansion vessel. This one needs an engineer.
5. Sludge or debris has caught in the seat
Older systems build up magnetite sludge over time. A fleck can lodge in the PRV seat and stop it closing fully. This often shows up as intermittent leaking that becomes constant.
Fix: a power flush cleans the system, and the PRV is replaced while the boiler is drained.
6. The system has been over-pressurised
If a previous top-up went past 2.5 bar without anyone noticing, the PRV will have opened and may not have reseated cleanly. Bringing the pressure back to 1 bar does not help if the seat is now damaged.
Fix: replace the PRV.
What you can safely check yourself
- Is the drip constant or only when the heating is running? Constant points at the valve seat or the filling loop. Only-when-hot points at the expansion vessel.
- What does the gauge read? Above 2.5 bar cold is too high. Below 1 bar cold is too low. Anywhere between 1 and 2 bar cold is normal.
- Is the filling loop fully closed? Trace the flexible pipe from the boiler and confirm the taps at both ends are shut.
- Is anything else in the system leaking? If the PRV is dumping water and the pressure stays stable, water is entering the system from somewhere: a radiator valve, a joint, the boiler itself.
When the boiler cuts out with a low-pressure fault
A PRV that has been leaking for a while eventually empties the system enough for the boiler to lock out on low pressure. Vaillant shows F22. Worcester Bosch shows a similar low-pressure code. The boiler is protecting itself by refusing to fire until the pressure is restored, which is the "safety switch off low water pressure" behaviour you may have read about.
Topping the pressure up will get it running again for a while, but if the PRV is faulty, it will just keep dumping. The permanent fix is to find the underlying cause. Our guide on why your boiler keeps losing pressure covers the wider pattern, and the Vaillant F22 fault code page walks through the pressure-loss side in more depth.
When to call a Gas-Safe engineer
Book an engineer if:
- The PRV is dripping continuously and does not stop
- You have topped the pressure up more than a couple of times in a week
- The pressure jumps sharply when the heating fires
- You cannot identify the cause from the checks above
- The boiler has cut out with a low-pressure or F22 fault more than once
Advanced Heating & Plumbing offers boiler repair in Watford, and covers the wider Hertfordshire, London and Middlesex area. Gas-Safe registered. Engineer on site within 2 hours, no separate call-out fee, and our engineers carry common PRVs, filling loops and expansion vessels on the van. Most valve issues are diagnosed and fixed on the first visit.
Book a Gas Safe boiler repair or call 01923 675246. Prices quoted in advance. The price you agree is the price you pay. If your boiler is due one, an annual boiler service catches PRV wear and expansion-vessel pressure loss before either becomes a fault.
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