A small pipe dripping outside near your boiler is usually the pressure relief valve (PRV) discharge, and it drips because system pressure has climbed too high or the valve's seat has worn. Common causes include an open filling loop or a failing expansion vessel. Never cap or block this pipe yourself, since it's a safety feature. Get a Gas Safe engineer to check the valve and vessel if the drip continues.
If you've spotted a small pipe dripping or trickling water low down on an outside wall, usually somewhere near where the boiler sits inside, this is almost always the discharge from a pressure relief valve doing exactly what it's designed to do. That doesn't mean ignore it, though. This guide explains what the valve is, why it discharges, what usually causes it, and the one thing you should never do about it.
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What the pressure relief valve does
Every sealed central heating system, the kind found on most modern combi and system boilers, has a maximum safe working pressure. The pressure relief valve is a safety device fitted to protect against that limit being exceeded. If pressure inside the system climbs above the valve's set point, it opens automatically and releases a small amount of water through a discharge pipe, usually routed to terminate outside the property at low level, well away from doors and windows.
It's a safety feature working correctly, not a fault by itself. The fault, when there is one, is usually whatever is pushing the pressure up high enough to trigger the valve in the first place, or wear inside the valve that's letting it weep even when pressure is normal.
Why the discharge pipe is dripping
A dripping or trickling PRV discharge pipe generally comes down to one of these:
- Genuinely high system pressure, often because a filling loop used to top up the system was left slightly open, letting mains pressure keep feeding in.
- A worn valve seat, where debris or age has stopped the valve sealing fully even at normal pressure, so it weeps continuously rather than only under genuine over-pressure.
- A failing expansion vessel, which can no longer absorb the normal expansion of water as it heats, so pressure spikes higher than it should during the heating cycle.
- Overfilling the system beyond its normal cold pressure when last topped up.
Boiler cold pressure typically sits around 1 to 1.5 bar depending on the manufacturer, and it's worth checking your boiler's own gauge against the manual before assuming anything is wrong.
Why you should never cap it
If a drip is a nuisance, marking a wet patch on brickwork or leaving a small drip stain, the answer is to fix the underlying cause of the discharge, not to interfere with the pipe itself. Any adjustment to the valve, the filling loop or the wider pressure system on a gas boiler should be carried out by a Gas Safe registered engineer.
The expansion vessel connection
Central heating water expands as it heats up, and the expansion vessel exists to absorb that extra volume, keeping the whole system within its normal pressure range through the heating cycle. Inside the vessel, a rubber diaphragm separates water on one side from a pocket of pressurised air or nitrogen on the other, and that air charge is what gives the diaphragm somewhere to flex into.
If the diaphragm splits or the air charge slowly leaks away over years, the vessel stops doing its job. Water that should be absorbed pushes system pressure up instead, and the pressure relief valve ends up discharging more often as a direct result. A PRV that drips repeatedly, especially if it seems to coincide with the heating firing up, is a common early sign of a tired expansion vessel rather than a fault with the valve itself, and it usually means the vessel needs recharging or replacing rather than any adjustment to the valve.
What to do about a dripping PRV
- Check the boiler pressure gauge against the manufacturer's normal range for a cold system, typically 1 to 1.5 bar.
- Make sure the filling loop, if fitted, is fully closed and not left slightly open after a top-up.
- Note whether the dripping happens constantly or only when the heating is actively running, since that timing is a useful clue for whoever comes to look at it.
- If pressure reads normal and the valve still drips, or pressure keeps climbing on its own, book a Gas Safe engineer to check the valve, filling loop and expansion vessel together rather than replacing parts by guesswork.
If the boiler is also losing pressure repeatedly rather than just discharging occasionally, our guide to why a boiler is leaking water covers the wider set of possible causes beyond the PRV alone.
Older boilers in Cornwall and Devon homes
Plenty of properties across Cornwall and Devon, particularly older cottages and converted barns, have combi boilers and expansion vessels that have quietly done their job for well over a decade without anyone giving them a second thought. Vessels of that age are exactly the ones most likely to have lost some of their air charge, so a PRV that starts dripping on a system that's never given trouble before is often simply that vessel finally reaching the end of its working life. We see this pattern most often as the weather turns colder and heating systems that have been idle over summer come back into regular use.
Frequently asked questions
What is the pipe dripping outside my house near the boiler?
It's almost certainly the discharge pipe for your boiler's pressure relief valve, sometimes called a PRV. It usually exits low down on an outside wall near where the boiler sits, and its job is to release water safely if system pressure climbs too high, protecting the boiler and pipework from damage.
Why is my pressure relief valve dripping?
Usually one of two things: the system pressure is genuinely too high, often because a filling loop was left slightly open after topping up, or the valve's internal seat has worn and is letting water through even at normal pressure. An expansion vessel that has lost its charge can also push pressure up enough to trigger it.
Can I cap or block the PRV discharge pipe myself?
No. Never cap, plug or block a pressure relief valve discharge pipe. It exists specifically to release pressure safely, and blocking it removes that safety function, which can lead to serious damage or a dangerous pressure build-up in the system. Any work on the valve itself should go through a qualified engineer.
Is a dripping PRV a gas safety issue?
A dripping discharge pipe on its own is a plumbing and heating fault, not a gas emergency, and doesn't need the gas emergency line. However, because the valve and its pipework connect directly to the boiler, any adjustment or repair to the valve itself should be carried out by a Gas Safe registered engineer.
How is a dripping PRV connected to the expansion vessel?
The expansion vessel absorbs the extra volume water takes up as it heats, keeping system pressure within its normal working range. If the vessel's internal diaphragm has failed or lost its charge, it can no longer absorb that expansion properly, so pressure spikes higher than it should and the pressure relief valve opens to compensate.
What should I do if the PRV discharge pipe keeps dripping?
First check your boiler pressure gauge against the manufacturer's normal range, typically around 1 to 1.5 bar when cold, and make sure the filling loop is fully closed. If pressure is genuinely high or the dripping continues at normal pressure, get a Gas Safe engineer to check the valve, the expansion vessel and the wider system.
Could the dripping actually be a hidden leak, not the PRV working normally?
It's worth confirming the water is coming from the discharge pipe itself and not tracking down from elsewhere, such as a leaking pipe joint higher up the wall. If the boiler keeps needing repressurising alongside the dripping, that combination points to water escaping somewhere in the system, which non-invasive leak detection can help locate.
PRV dripping and pressure keeps dropping? Let's find out why
If your boiler keeps losing pressure alongside a dripping discharge pipe, we'll check the heating circuit for a hidden leak across Cornwall and Devon, with minimal disruption.
Call Dickie on 07822 025 911 No Find, No Fee on residential leak detection (subject to terms)