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Boiler Pressure relief valve: What It Does and Signs It's Failing

The pressure relief valve is a spring-loaded safety valve inside the boiler, connected to a copper discharge pipe that runs out through the wall. If the heating water ever reaches a dangerous pressure, it opens and dumps water outside. Ideal presets the one on its Logic Combi2 C to 3 bar, and it can't be adjusted.

How it works

A spring holds a seal shut against the system pressure. Once pressure beats the spring, the valve lifts and lets water out until pressure falls back, then it closes again. A bit of grit or scale on the seal can stop it closing fully after it has opened.

Signs it is failing

• Drips or a damp stain under the copper pipe on the outside wall
• Boiler pressure dropping with no leak anywhere in the house
• Valve letting water go even though the gauge never reaches 3 bar
• Water running from the discharge pipe every time the heating heats up

A valve that weeps shows up as repeated low-pressure codes. If it only opens when the system is hot, the real cause is usually the expansion vessel or overfilling.

Can you replace it yourself?

You can look at the discharge pipe outside and check the gauge, nothing more. Vaillant's user manual says not to alter the relief valve or any other safety device, and Baxi says cleaning or replacing it is for a Gas Safe registered engineer. Never cap or block the discharge pipe, since that's the escape route if pressure gets too high.

Buying the right part

Relief valves come in different pressure ratings and fittings, so the replacement has to be the one listed for your boiler model.

Error codes linked to the pressure relief valve

• E118Low water pressure in the heating circuitBaxi
• E119Low primary water pressure (Platinum Combi)Baxi
• F.22Water pressure in the heating system is too lowVaillant
• CE 207 / H07Heating system water pressure too lowWorcester Bosch

Last reviewed 2026-10-21.