The Pressure Relief Valve (PRV) Explained
The pressure relief valve is your boiler’s last line of defence against dangerous over-pressure. Learn what it does and why a dripping PRV is never to be ignored.
Of all the parts on your boiler, the pressure relief valve is the one that exists purely for safety. It is designed to open and release water if system pressure ever climbs to a dangerous level. A PRV that drips, or one that someone has plugged, is a serious matter — here is why.
What it does
The PRV (also called a T&P or safety relief valve) is a spring-loaded valve set to open at a fixed pressure — commonly 30 psi on residential systems. If pressure reaches that point it opens and discharges water, preventing the kind of over-pressure that can rupture a vessel.
Why a PRV drips
- Genuine over-pressure — often a failed expansion tank or an overfilling fill valve.
- A worn valve seat that no longer seals, even at normal pressure.
- Debris under the seat after it has discharged once.
The one thing never to do
Never cap, plug or screw down a relief valve to stop it dripping. It is the system’s last safety device. The correct response is to find why it is discharging — usually pressure-related — and fix that cause, then replace the valve if its seat is damaged.
Key takeaways
- The PRV is a pure safety device that opens to release dangerous over-pressure.
- A dripping PRV usually points to a failed expansion tank or an overfilling system — not just a bad valve.
- Never plug or disable a relief valve; diagnose the pressure cause instead.
Frequently asked questions
Is it safe to use a boiler if the relief valve is dripping?
A slow drip is a warning, not an immediate danger, but it should be diagnosed promptly. If the valve is actively discharging hot water or steam, switch the boiler off and call a licensed technician — pressure has reached the safety limit for a reason.
Can I just replace the relief valve myself?
Swapping the valve without fixing the underlying pressure problem only buys a few weeks. Because it involves the pressurised, hot side of a gas appliance and BC gas-safety rules, this is work for a licensed technician.
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