Pressure Relief Valve London
Boiler PRV dripping or discharging? We replace pressure relief valves same day across London — and fix the underlying cause so the new one doesn't start dripping again.
What Your Boiler Pressure Gauge Is Telling You
A capped PRV means the boiler or cylinder is effectively operating without its primary safety device. This is a serious hazard — a sealed system at extreme pressure can rupture. If your PRV discharge pipe has been capped, call us immediately. We see this in London buy-to-let properties where tenants or unqualified workers have tried to stop the drip by blocking the pipe.
Types of Pressure Relief Valves
Boiler PRV (3 bar)
The most common PRV found on sealed central heating systems. Set at 3 bar — discharges if system pressure exceeds this. Connected via discharge pipe to a safe outlet. If dripping at normal operating pressure (1.0–2.5 bar), the PRV seal has failed.
£85–£150 inc. valve
Cylinder T&P Relief Valve
Temperature and pressure relief valve on unvented hot water cylinders (Megaflo, Heatrae Sadia, etc.). Responds to BOTH excess pressure AND excess temperature. Set at 90°C and 8–10 bar. If discharging, check cylinder thermostat and high-limit stat — do NOT simply replace without investigating.
£95–£180 inc. valve
Pressure Reducing Valve (PRV)
Not a relief valve but often confused with one. A PRV reduces incoming mains pressure from Thames Water (which can reach 8–10 bar in London) to a safe system pressure (typically 3 bar). A failed PRV can allow full mains pressure into the system, over-stressing all fittings.
£120–£220 inc. valve
Expansion Vessel (linked to PRV)
A failed expansion vessel diaphragm causes the boiler PRV to repeatedly lift as pressure rises above 3 bar. Replacing the PRV without addressing the expansion vessel causes the new valve to also start discharging. We always check expansion vessel pre-charge pressure.
£180–£320 inc. vessel
PRV Diagnosis Guide
| Symptom | Likely cause | Fix |
|---|---|---|
| PRV drips constantly at all times | PRV seal has failed — valve weeping at normal pressure | Replace PRV £85–£150 |
| PRV only discharges when system heats up | Expansion vessel failed — system pressure rises above 3 bar on heating | Replace expansion vessel £180–£320 + PRV if damaged |
| PRV discharge pipe running constantly | Filling loop left open or filling loop PRV faulty — system continuously gaining pressure | Close filling loop, check filling loop valve £45–£85 |
| Cylinder T&P valve discharging | Overheat or overpressure in cylinder — check cylinder thermostat and high-limit stat | Thermostat replacement £95–£180 · Urgent — do not ignore |
| PRV discharge pipe capped or blocked | Previous plumber or tenant capped discharge — safety hazard | Emergency: remove cap, assess underlying fault — URGENT |
| PRV whistling but not fully discharging | Partial seat failure — valve opening at slightly below set pressure | Replace PRV £85–£150 — do not delay |
Pressure Relief Valve Prices — London 2026
| Job | Time | Price |
|---|---|---|
| Boiler PRV replacement | 45–75 min | £85–£150 inc. valve |
| Cylinder T&P relief valve replacement | 1–2 hrs | £95–£180 inc. valve |
| Pressure reducing valve (PRV on mains) replacement | 1–2 hrs | £120–£220 inc. valve |
| System pressure diagnosis | 1 hr | £95 (deducted from repair) |
| PRV + expansion vessel (same visit) | 2–3 hrs | £250–£420 combined |
| PRV discharge pipe extension / reroute | 1–2 hrs | £85–£150 inc. fittings |
Fixed quote before starting. Root cause diagnosis included. No call-out fee. 7 days a week.
Pressure Relief Valve Repairs Across London
Pressure Relief Valve FAQs
Why is my pressure relief valve dripping?
Either the system pressure is too high (the PRV is doing its job — check for a failed expansion vessel), or the PRV seal itself has failed and is weeping at normal pressure. Never just replace the PRV without finding the cause — the new one will also start dripping if pressure remains too high.
How much does pressure relief valve replacement cost in London?
Boiler PRV replacement: £85–£150 including the valve. Cylinder T&P valve: £95–£180. If a failed expansion vessel is also causing the problem, adding expansion vessel replacement (£180–£320) at the same visit saves a second call-out.
Is a dripping pressure relief valve dangerous?
The valve itself is a safety device — a dripping PRV means it's working. The real danger is a capped or blocked PRV in an over-pressure system, which can cause a rupture. If your PRV has been capped or tied off, call us immediately.
Where does the PRV discharge pipe go?
Under Part G of Building Regulations, the discharge pipe must terminate visibly — over an external gully, a sink, or via a tundish. It must never be capped or discharged where backflow could occur. Any discharge must be observable so faults are spotted.
Why does my boiler pressure keep rising?
Repeatedly rising pressure causing PRV discharge is almost always a failed expansion vessel diaphragm. Without a working expansion vessel, thermal expansion has nowhere to go and pressure builds above 3 bar on every heating cycle. Fix: expansion vessel replacement £180–£320 or re-gas £85–£120.
Can I just replace the PRV myself?
You can, but if you replace it without finding the root cause it will simply start dripping again. More importantly, we diagnose the reason the PRV failed — if it's a failed expansion vessel or faulty thermostat, those are higher-priority fixes than the PRV itself.
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PRV Dripping? We'll Fix the Root Cause Today.
From £85 · Same day · Root cause diagnosis · No call-out fee
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