Pressure relief valve

1 What it does

Vaillant describes a spring held closed against the system's pressure. When pressure beats the spring, the valve opens and lets hot water or steam out by a separate route, and the spring closes it once pressure falls¹. That stops a build-up that could shut the boiler down or, in extreme cases, cause a rupture¹.

Different makers name different figures. Baxi's Platinum guide and Main's Eco Compact guide both say 3 bar²⁴. Vaillant's 2023 ecoTEC plus manual warns that it may open at 2.5 bar if the expansion vessel is no longer compensating³.

2 If water is coming from the pipe

Some discharge is expected at times, since releasing water is this valve's job, but Vaillant says a constant or excessive leak means the boiler keeps reaching high pressure, and should go to an engineer¹. The pipe usually ends outside on a wall or low down, so look there at ground level¹. Water leaving will lower your gauge; see pressure keeps dropping.

What the manuals ask differs:

• Worcester 4000: a leaking PRV pipe in normal use can be a sign of high pressure, so bleed a radiator, as that manual describes⁵.
• Baxi Platinum: a reading of 3 bar or more means a prompt call to your installer or engineer².
• Main Eco Compact: disconnect a temporary filling loop and contact your installer⁴.

For a leak, Vaillant's website advises shutting off the water supply and heating and then calling an engineer, with a faulty vessel or passing filling loop among likely sources⁶. Its valve page also names sludge and limescale on the seal, both for a professional¹. Reading the gauge first gives the engineer a useful fact, and pressure too high covers those readings.

3 Who replaces it

Vaillant says it isn't safe or recommended to try this yourself¹. On a gas boiler, the valve usually sits inside the casing, so this is work for a Gas Safe registered engineer under the rules set out at who can work on a boiler⁷.

4 The cylinder has its own

An unvented hot water cylinder carries separate safety devices, and Approved Document G for England asks for a marked warning on it: flow from that discharge pipe means heater off and a call to the installer⁸. See hot water cylinder.