Boiler pressure relief valve dripping
Check the steps below first — if you\'re not confident, get it fixed safely today.
Find a Gas Safe engineer near you →⚠️ 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.
Not sure if this matches your problem?
Use our interactive tool — answer a few questions and get a personalised diagnosis.
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.
This job needs a qualified engineer — post it now
Post the job and we'll match you with vetted local engineers. Free, no obligation.
Find a Gas Safe registered engineer near you and get it fixed properly.
Was this guide helpful?