Boiler pressure relief valve dripping
Check the steps below first — if you're not confident, get it fixed safely today.
Post a job — we'll find you an engineer →⚠️ The pressure relief valve (PRV) is a safety device. Do not cap off or block the discharge pipe — it prevents boiler explosion under overpressure conditions. If the PRV discharges scalding water, do not stand near the discharge pipe.
A dripping pressure relief valve (PRV) on your boiler is one of those problems that catches many homeowners off guard — you notice water trickling from a small pipe on the outside wall and wonder if something's seriously wrong. The good news is that the PRV is actually doing its job as a safety device, designed to release excess pressure before it becomes dangerous. However, continuous dripping isn't normal and usually signals one of three issues: a failed expansion vessel that can't handle the pressure surge as your heating system warms up, a filling loop left partially open that's continuously pressurising the system, or debris trapped under the PRV seat itself. This guide walks you through diagnosing which culprit is responsible so you know whether it's a quick fix or time to call a Gas Safe engineer.
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Most likely cause & what to check
The boiler PRV is a brass valve set to open at 3 bar. It discharges through a pipe to outside the property. Occasional small amounts of water from this pipe when the boiler heats up is normal. Continuous or frequent dripping is not.
Check the system pressure gauge — if it consistently reads above 2 bar when the system is cold, the expansion vessel has likely failed and cannot absorb the pressure increase as the system heats.
Check the filling loop — confirm both valves on the filling loop under the boiler are closed when not in use. A partially open filling loop continuously adds mains pressure to the sealed system, causing overpressure.
A failed expansion vessel requires a Gas Safe engineer to replace. The engineer will also need to repressurise the vessel with a bicycle pump before fitting.
If the expansion vessel is fine and the filling loop is closed, the PRV itself may have debris under its seat causing it to weep. A new PRV is inexpensive (£15–£30) but must be fitted by a Gas Safe engineer.
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Frequently asked questions
Is it dangerous if my PRV keeps dripping?
The PRV dripping is actually the safe outcome — it's releasing overpressure before it damages the boiler. However, continuous dripping isn't normal and indicates an underlying fault that needs addressing. Ignoring it won't cause an explosion, but it wastes water and suggests your heating system isn't working as intended.
How do I know if my expansion vessel has failed?
Check the pressure gauge on your boiler when the system is completely cold (first thing in the morning is ideal). If it reads consistently above 2 bar, the expansion vessel has likely failed and lost its ability to absorb the pressure increase as water heats and expands. A Gas Safe engineer will need to replace it.
Can I just cap off the PRV discharge pipe to stop the dripping?
No — absolutely do not block or cap the discharge pipe. The PRV is a critical safety device that prevents your boiler from overpressurising and potentially exploding. Capping it removes that protection entirely. Always address the root cause instead.
What's a filling loop and how do I check if it's the problem?
The filling loop is a pair of valves under your boiler that connects the mains water supply to your heating system — it's used only to top up pressure when needed. Both valves should be closed when you're not actively filling the system; if either is left partially open, it continuously pressurises the system and forces the PRV to weep. Turn both valves fully clockwise to close them.