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Pressure Relief Valve — The Boiler's Safety Device
The PRV is a critical safety device on sealed heating systems. It automatically opens to release water if system pressure exceeds a safe level, preventing dangerous over-pressurisation.
🔍 Where to find it
Usually a brass valve on the boiler or nearby pipework, with a small pipe running from it — discharging outside, into a drain, or into a bucket. This discharge pipe is sometimes the only sign the PRV has operated.
⚙️ How it works
The PRV opens at a set pressure — usually 3 bar. If system pressure rises above this, it releases water until pressure drops. You should never regularly see water dripping from the PRV discharge pipe. Occasional drips after repressurising are normal. Regular dripping means there's an underlying pressure issue.
🛠 What to tell an engineer
"Water is dripping from the discharge pipe near my boiler on a regular basis." A Gas Safe engineer will check the expansion vessel, system pressure, and PRV. Do not ignore a weeping PRV.
Important: Never replace or bypass a PRV yourself. It is a safety device. A weeping PRV means something else in the system needs attention. Always call a Gas Safe engineer.
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General guidance only — not a substitute for professional advice. Always consult a qualified engineer if you are unsure. Gas work must only be carried out by a Gas Safe registered engineer. Unvented (pressurised) hot water systems must be worked on by a G3-qualified engineer. We accept no liability for any loss, damage, or injury arising from the use of this information.
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