Pressure Relief Valve
/ ˈprɛʃə rɪˈliːf valv /
Also known as: PRV, safety relief valve, pressure safety valve, relief valve
Definition
A pressure relief valve (PRV) is a spring-loaded safety device that automatically opens to discharge water when system pressure exceeds a preset limit (typically 3 bar on a sealed heating system), then reseats when pressure drops. Found on sealed central heating systems (on the boiler), unvented hot water cylinders (on the cold inlet and as a combined temperature and pressure relief valve on the cylinder body), and pressurised vessels. A regularly dripping PRV almost always indicates a failed or under-charged expansion vessel - not a faulty valve. Capping or bypassing a PRV is extremely dangerous. Discharge must be piped safely away via a tundish.
In practice
The most dangerous response to a dripping PRV is to cap or plug the discharge pipe. This is occasionally done by well-meaning but uninformed property owners or unqualified contractors to stop the nuisance drip. A capped PRV cannot discharge if system pressure rises dangerously - the result can be a catastrophic vessel failure or explosion. PRV discharge pipes must always terminate in an open, safe position. If a PRV is dripping, the correct action is to identify and fix the root cause (failed expansion vessel being the most common) not to suppress the symptom.
PRVs should be exercised (manually lifted open by the test lever or cap) annually as part of boiler and system servicing, to confirm they move freely and reseat cleanly without dripping after operation. A PRV that has repeatedly opened (due to an overpressure event) may not reseat cleanly and may drip continuously even after the overpressure cause is fixed - in this case, the PRV itself must be replaced. PRVs have a finite service life and should be replaced every 5-10 years as part of routine maintenance regardless of condition, as the internal spring can weaken over time.