PRV dripping —
valve, pressure
or washer problem?
A pressure relief valve that is dripping may have a failed seat, may be responding to system overpressure, or may have been damaged by a previous lift. Each requires a different response. A new washer addresses only the sealing element. A new valve addresses only the seat condition. Neither fixes an expansion vessel problem or an overfilled system — and if the cause is system pressure, the new part will be subjected to the same condition.
Safety scope: This article is for identifying likely water-side sealing and pressure-system symptoms. Do not open combustion chambers, gas-side components, flues or any appliance you suspect is unsafe. Gas work, boiler safety checks and unsafe appliance decisions belong with a competent Gas Safe registered engineer.
This article is a diagnostic reference for identifying the cause of a dripping boiler PRV. Any work on boiler components, gas appliances, or safety devices must be carried out by a Gas Safe registered engineer. A discharging PRV is a safety event — do not bypass or cap it.
The four main causes — and why they look similar
Failed valve seat
The PRV no longer seals correctly at normal operating pressure. Water passes through the valve body even when system pressure is within the correct range. Dripping is present at cold pressure, not only when the boiler fires. A new valve addresses this — but only if this is actually the cause.
System overpressure — expansion vessel
The expansion vessel has lost its pre-charge or the diaphragm has failed. On heating, system pressure rises beyond the PRV set point. The valve is working correctly — it is discharging because the pressure is too high. A new valve does not fix this.
Overfilled system
The system was filled to too high a cold pressure. On heating, normal thermal expansion takes the pressure above the PRV set point. The valve discharges correctly. Reducing system pressure — not replacing the valve — is the response.
Previous lift — damaged seat
A PRV that has been manually lifted or has discharged at high pressure may not reseat cleanly. The seat is damaged by the discharge event. The valve drips continuously at normal pressure afterward. This is a valve condition problem — but the original cause of the discharge also needs to be investigated.
The key diagnostic question is: at what system pressure does the PRV drip? If the system pressure gauge reads within the normal operating range (typically 1.0–1.5 bar cold) and the PRV is dripping, the valve seat condition is the more likely cause. If the system pressure is at or above the PRV set point (typically 3 bar), the pressure is the cause — not the valve condition. Check the pressure gauge before concluding the valve has failed.
Drip from a failed seat versus discharge from overpressure
Failed seat — dripping at normal pressure: the PRV drips or weeps continuously, including when the boiler is cold and the system pressure is within the normal range. The drip is present regardless of system temperature. Re-tightening has no effect. The drip may be slow but is consistent. This is a valve condition — the internal seat is not sealing against the valve body at the normal operating pressure.
Overpressure discharge — PRV working correctly: the PRV discharges specifically when the boiler fires from cold, or after a period of sustained heating. The system pressure gauge rises to or above the PRV set point before or during the discharge. The PRV may be dry at cold start and begin to drip or discharge as the system heats up. This is not a valve fault — the valve is functioning as intended in response to excessive system pressure.
Distinguishing between these two requires observing the system pressure gauge behaviour in relation to the discharge. A Gas Safe registered engineer should carry out this assessment and any subsequent work.
When the expansion vessel is the cause
The expansion vessel absorbs the increase in system water volume as the heating system heats up. If the vessel has lost its air pre-charge — because the pre-charge, vessel size or usable gas volume is no longer correct for the system — the vessel has less expansion capacity. On every heating cycle, the system pressure rises above what the vessel can accommodate, and the PRV discharges to relieve it.
The symptoms are specific: the PRV discharges when the boiler fires, the system pressure rises rapidly on heating, and the vessel may feel heavy. Pressing the Schrader valve on the vessel air side may produce no air — or water, if the diaphragm has failed. A Gas Safe registered engineer should check the vessel pre-charge pressure and advise on recharging or replacement.
Fitting a new PRV does not resolve an expansion vessel problem. The new valve is immediately subjected to the same overpressure condition on the next heating cycle.
When the system was overfilled
A cold fill above the boiler manufacturer's specified range reduces the available margin to the PRV set point, but 1.5 bar is not a universal fault threshold and does not by itself guarantee discharge. Check static head, pre-charge, vessel sizing and the actual hot-pressure rise. This is common immediately after a system refill, after a filling loop fault, or where the system was inadvertently pressurised above the correct cold fill level.
The system pressure gauge at cold should read approximately 1.0–1.5 bar in most domestic systems — the correct value depends on the system height and design. If the cold pressure is significantly above this, the system may have been overfilled. Reducing the cold fill pressure to the correct value — with appropriate isolation and using the correct draining procedure — may resolve the PRV discharge without any valve work. This should be assessed by a Gas Safe registered engineer.
Why a new washer often does not fix the drip
- The cause is system pressure, not the valve sealing element. If the system is overpressurising the PRV on every heating cycle, a new washer is subjected to the same overpressure. It may seal correctly at cold pressure and discharge again when the system heats — the same symptom, the same apparent failure.
- The valve seat is damaged, not just the sealing element. A PRV that has discharged at high pressure may have a damaged internal seat surface. A new washer or sealing element pressed against a damaged seat may not produce a reliable seal. The valve body itself requires replacement.
- The valve was operated manually. Manually lifting a PRV to test it or to relieve pressure can damage the internal seat. The valve may not reseat cleanly after manual operation. This is a known risk of manual PRV operation — the valve may need replacing after being lifted.
A PRV that has been discharging should not be capped, bypassed or blocked. The PRV is a safety device — it protects the system against dangerous overpressure. If a discharging PRV is inconvenient, the cause of the overpressure must be addressed, not the discharge route. Any work on the PRV or on the system pressure and expansion vessel must be carried out by a Gas Safe registered engineer.
Identify the cause before replacing any part.
A dripping PRV has four possible causes — failed seat, expansion vessel fault, overfilled system, or post-lift seat damage. Each requires a different response. Check the system pressure gauge before concluding the valve has failed. Observe whether the drip is present at cold pressure or only on heating. Check the expansion vessel condition. A Gas Safe registered engineer should assess the system pressure behaviour, the vessel condition, and the valve — and advise on the correct repair sequence.
FAQ
Why is my boiler pressure relief valve dripping?
A dripping pressure relief valve has several possible causes that require different responses. Common causes include: the system pressure is too high — from an overfilled system, a waterlogged expansion vessel, or an expansion vessel that has lost its pre-charge — and the PRV may be discharging in response to overpressure; the PRV seat has failed — the valve no longer seals correctly at normal operating pressure, and water passes through the valve body even when system pressure is within the correct range; or the PRV was manually operated or lifted at some point, and the valve seat has not reseated cleanly. Replacing the valve addresses a seat failure but does not resolve an overpressure cause. Reducing system pressure addresses an overpressure situation but does not repair a damaged valve seat.
Is it normal for a PRV to drip occasionally?
A small amount of water at the PRV discharge point after the boiler fires from cold — particularly in a system that is at the upper end of its normal pressure range — may indicate the pressure is briefly exceeding the PRV set point during thermal expansion. This is more likely where the expansion vessel has partially lost its pre-charge. It is not normal for the PRV to drip continuously at cold system pressure, or to discharge significant volumes of water. Any persistent dripping or regular discharge should be investigated by a qualified heating engineer, and gas boiler work should be handled by a Gas Safe registered engineer — it indicates either a system pressure problem or a valve seat condition that requires attention.
Does a new washer fix a dripping PRV?
A new sealing element may reduce or stop the drip if the sealing element is faulty or the valve seat is only marginally damaged — but if the underlying cause is system overpressure, the new washer will be subjected to the same overpressure condition and may lift again. If the expansion vessel is waterlogged, the system will continue to overpressurise on every heating cycle, and the PRV will continue to discharge regardless of its condition. Addressing the sealing element alone is not a complete fix where the system pressure is the primary cause. Both the pressure cause and the valve condition should be assessed before treating the washer as the fault.