Unvented cylinder 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 →⚠️ Do not cap, plug, or restrict the pressure relief valve discharge pipe. This pipe is a critical safety device — blocking it risks catastrophic cylinder failure. Call a qualified unvented cylinder engineer.
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
On an unvented hot water cylinder (Megaflo, Heatrae Sadia, Gledhill, etc.), a small pipe exits the cylinder and discharges to a safe location (usually outside or into a tundish). If this pipe is dripping or running with water, the pressure relief valve (PRV) is opening — this is a safety warning, not a normal condition.
The PRV opens to release excess pressure when system pressure exceeds its rated setting (usually 6–8 bar). Persistent dripping means the cylinder pressure is regularly exceeding this threshold. Common causes: the expansion vessel has failed (the most common cause), the pressure reducing valve (PRV) is set too high, or the incoming mains pressure is too high.
The expansion vessel is a small pressurised vessel (inside the cylinder on combination units, or external on some cylinders) that accommodates the expansion of water as it heats from cold to 60°C. If the vessel membrane has failed, the vessel fills with water and cannot accommodate expansion — pressure spikes on every heat cycle.
An engineer will: check the expansion vessel charge pressure using a tyre pressure gauge (should match the incoming cold pressure, typically 3 bar), inspect the vessel membrane, and replace the vessel if failed. An expansion vessel replacement typically costs £150–£300 fitted.
If the PRV is dripping only occasionally and the expansion vessel is intact, the PRV itself may be failing and weeping at normal pressure. PRV replacement is straightforward (£30–£60 part) but requires the cylinder to be drained down.
Unvented cylinders must only be worked on by engineers with G3 (or equivalent) qualifications — this is a legal requirement under Building Regulations Part G. Attempting to repair an unvented cylinder without the correct qualification is illegal and extremely dangerous.
This job needs a qualified engineer — post it now
Post the job and we'll match you with vetted local engineers. Free, no obligation.
Was this guide helpful?
Frequently asked questions
Why should I never cap or block a PRV discharge pipe?
The pressure relief valve is the last line of safety defence on an unvented cylinder. If pressure builds beyond safe limits and the PRV cannot open, the cylinder can fail catastrophically — the energy release from a failed unvented cylinder is similar to a small explosion. A dripping PRV is a warning to investigate the cause, not an excuse to silence it.
Who can work on my unvented hot water cylinder?
Only engineers with G3 Unvented Hot Water qualification or equivalent. This is a legal requirement under Building Regulations Part G3. Domestic gas engineers are not automatically qualified to work on unvented cylinders — check that whoever you hire holds the correct qualification.
How much does an expansion vessel replacement cost on an unvented cylinder?
An expansion vessel replacement on an unvented cylinder typically costs £150–£350 fitted. If the PRV itself needs replacing (£30–£60 part) that is usually done at the same visit. Always use a qualified G3 engineer for this work.