Unvented cylinder pressure relief valve dripping

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.