Boiler leaking from the pressure relief valve

The pressure relief valve (PRV) on a central heating boiler is a safety device that opens and discharges water when the system pressure exceeds 3 bar, preventing dangerous over-pressurisation. A drip or trickle from the discharge pipe (usually a 15mm copper pipe running outside or into a tundish) indicates the PRV has operated.

The most common cause of a PRV discharging is a failed expansion vessel. In a sealed system, the expansion vessel absorbs the increase in water volume as the system heats up. If its internal diaphragm has failed, the system pressure rises unchecked until the PRV opens.

A Gas Safe engineer can test the expansion vessel with a tyre pressure gauge on the Schrader valve — it should read 0.75–1 bar when the system is cold and depressurised. A zero reading means the diaphragm has failed. Re-pressurising the vessel (adding nitrogen or air) may be a temporary fix; replacement is usually more reliable.

A second cause is an overfilled system — if someone has been repeatedly topping up the pressure via the filling loop, the system water volume is too high, leaving no room for thermal expansion. The engineer will drain down to the correct pressure (1–1.5 bar cold) and check the expansion vessel.

If the PRV itself is faulty (has been weeping for a long time), the seat can become damaged and it may leak even when pressure is normal. In this case the PRV itself needs replacing — a standard 3 bar PRV costs £10–£20 but fitting requires the system to be isolated and drained.

Do not ignore a continuously discharging PRV even if the boiler seems to be working. Repeated over-pressurisation stresses seals and heat exchangers and can cause premature boiler failure.