Pressure Relief Valve Dripping Outside: Why It Happens and What to Do in London

What the Pressure Relief Valve Does

The pressure relief valve (PRV) is a safety device fitted on all sealed central heating systems — combi boilers, system boilers, and unvented hot water cylinders. It is set to open at 3 bar (or in some cases 4 bar on commercial systems). When the system pressure exceeds the PRV set point, the valve opens and discharges water through the discharge pipe to a safe location outside the property — typically via a small copper or plastic pipe exiting the external wall at low level, discharging over a drain or gully.

The PRV is a last resort safety device. It opens when the system pressure exceeds the design limit to prevent boiler or cylinder damage or rupture. A PRV that is discharging water is telling you that the system pressure is rising above 3 bar at some point in the heating cycle. This is not normal operation — it is the PRV doing its job in response to a system fault.

Cause 1: Failed Expansion Vessel

The most common cause of PRV discharge in London homes is a failed expansion vessel. The expansion vessel normally absorbs the pressure increase caused by thermal expansion of the heating water. When the expansion vessel diaphragm perforates and the vessel becomes waterlogged, it can no longer absorb expansion. The system pressure rises above 3 bar when hot and the PRV opens. Water discharges from the external pipe until the pressure drops back below 3 bar, at which point the PRV closes. The pattern repeats every heating cycle.

The diagnosis is the Schrader valve test: with the system cold and depressurised at the vessel, press the Schrader valve pin on top of the expansion vessel. If water comes out, the diaphragm has perforated and the vessel must be replaced. If air comes out, the nitrogen pre-charge has bled off and the vessel can be re-charged to 1 bar — a much simpler and less expensive repair.

Cause 2: Overfilled System

If the heating system is overfilled — the cold fill pressure is above 1.5 bar — then normal thermal expansion of the water will push the pressure above 3 bar when hot, causing PRV discharge. This is most commonly caused by a faulty filling loop valve that is allowing mains water to slowly trickle into the sealed circuit. Over days or weeks, the cold pressure gradually rises. When the boiler fires, the already-elevated cold pressure rises further with thermal expansion, exceeding 3 bar.

The check is simple: read the pressure gauge when the boiler is cold and the system has not fired for several hours. The cold pressure should be 1.0 to 1.5 bar. If it is consistently above 1.5 bar, the system is being overfilled. Check the filling loop valves are fully closed. If the pressure continues to creep up with the filling loop valves closed, there is a fault in the valves and the filling loop should be replaced.

Cause 3: PRV Failure

A less common but possible cause is a PRV that has failed in the partially-open position — the valve seat has deteriorated and the valve now discharges at a lower pressure than its 3 bar set point. This is most likely in properties where the PRV has previously discharged frequently (because of an expansion vessel fault that was not repaired promptly) — repeated discharge cycles erode the valve seat. A PRV that discharges when the system is at normal operating pressure (below 3 bar) has failed and must be replaced.

What Not to Do

Do not cap the PRV discharge pipe or block the discharge. The PRV is a safety device and blocking its discharge creates a dangerous situation where system overpressure has no release path. Do not simply ignore a dripping PRV and rely on topping up the system with the filling loop — this is masking a fault, wastes water, and accelerates corrosion in the system from oxygen introduced by repeated refilling. Have the fault diagnosed and repaired. Prestige Engineers diagnose and repair PRV discharge across all London boroughs.