Glossary · Boilers and boiler types
What is a pressure relief valve?
A pressure relief valve is a safety valve on a sealed central heating system or boiler. If the pressure in the system climbs too high, it opens and lets water escape through a discharge pipe, usually to the outside of the house. It protects the boiler and the pipework from damage.
Key points
- It is a safety device. It opens only if the system pressure goes beyond a set limit.
- On the Worcester Greenstar combi we checked, the safety valve opens at 3 bar. The maximum operating pressure is 2.5 bar.
- Worcester warns that the discharge can be boiling water or steam, so the pipe must be routed away from doors, windows and electrics.
- Water coming from the outside pipe means the pressure went too high or the valve is faulty. Both need an engineer.
- The valve is part of the boiler, so any repair is work for a Gas Safe registered engineer.
How a pressure relief valve works
A sealed heating system is full of water under pressure, with an expansion vessel taking up the extra volume as the water heats. If something goes wrong, such as a vessel that has lost its charge or a system that has been overfilled, the pressure can keep rising. The pressure relief valve is the safety backstop. At a set pressure it opens and lets water out, and the pressure drops.
It is a spring-loaded valve and it normally stays shut. The Worcester Greenstar 24/28/30 combi manual calls it a safety device for the boiler and gives the figures for that model: a maximum permissible operating pressure of 2.5 bar, and a maximum system pressure of 3 bar at which the safety valve opens.[1]Worcester, Bosch Group, Greenstar 24/28/30 RSF combination boiler: Installation, commissioning and servicing instructions The user guide shows the best pressure on a cold system as between 1 and 1.5 bar.[2]Worcester, Bosch Group, Greenstar 24/28/30 RSF combination boiler: User instructions The gap between normal running and the valve opening is deliberate. Other boilers have other limits, so use the figures in your own manual.
The discharge pipe
The valve does not release water into the room. It is connected to a discharge pipe that runs to the outside. The Worcester installation manual says this pipe should be at least 15 mm copper, should run downwards and away from electrical equipment or other hazards, and should preferably end at an external drain or soakaway.[1]Worcester, Bosch Group, Greenstar 24/28/30 RSF combination boiler: Installation, commissioning and servicing instructions It also warns that if the valve opens it may discharge boiling water or steam, and that the outlet must not be sited where it could cause a hazard above a window, entrance or other public access.[1]Worcester, Bosch Group, Greenstar 24/28/30 RSF combination boiler: Installation, commissioning and servicing instructions
That is why a wet patch or a steady drip under the outlet pipe on an outside wall matters. It is the visible sign that the valve has operated.
Reading the signs
| What you see | What it may mean | What to do |
|---|---|---|
| Water dripping or running from the external discharge pipe | The valve has opened, or it is not sealing | Keep away from the pipe, switch the heating off and book a Gas Safe registered engineer |
| Pressure gauge climbing well above its usual range | A pressure problem, which the valve may be reacting to | Contact your installer or an engineer, as the user guide advises |
| Gauge keeps falling and you keep topping up | Water is being lost from somewhere | Book an engineer, as repeated top-ups are not a fix |
The Worcester user guide tells the user to contact their installer or maintenance engineer if a permanent, significant increase or decrease shows on the gauge, and says that if leaks are found a service engineer must be contacted to inspect the installation.[2]Worcester, Bosch Group, Greenstar 24/28/30 RSF combination boiler: User instructions A filling loop is how a low system is topped up. Our page on boiler leaking water covers other places a leak can show.
Who can work on it
The valve is part of the boiler’s heating circuit and sits inside the casing on this type of boiler. Anyone employed to work on gas appliances in a home must be Gas Safe registered and competent in that area of gas work.[3]Health and Safety Executive, Domestic gas: frequently asked questions That means a householder should not fit, adjust or block a relief valve. BASI’s boiler repair team deals with these calls.
Example
Helen in Otley sees a puddle under the pipe that comes out of her kitchen wall, and the boiler pressure gauge reads high. She keeps well clear of the pipe, switches the heating off and calls an engineer. The engineer finds the cause and decides whether the valve, the expansion vessel or something else needs attention. She does not try to plug the pipe, because the valve needs somewhere to discharge.
What people get wrong
Many people think a dripping relief pipe is a small plumbing leak that can wait. It is a sign that the safety valve has opened or is not sealing, and the discharge can be very hot. It deserves a prompt call to an engineer.
The second mistake is blocking or capping the discharge pipe to stop the drip. That removes the route the safety valve needs, and it is exactly what the safety device is there to avoid.
The bottom line
A pressure relief valve is a safety valve that stays shut until the system pressure goes too high. If water appears at the outside discharge pipe, treat it as a warning and not as a nuisance. Switch the heating off, keep clear of the pipe and have a Gas Safe registered engineer find out why the pressure rose.
Common questions
What does a pressure relief valve do?
It opens to release water if the pressure in a sealed heating system rises too far. That protects the boiler and pipes from damage. It is a last line of defence rather than part of normal running.
Why is water dripping from the pipe outside my house?
The pressure relief valve has probably opened. Worcester says the discharge can be boiling water or steam. Keep clear of the pipe, switch the heating off and call a Gas Safe registered engineer.
At what pressure does the valve open?
It depends on the boiler. On the Worcester Greenstar 24/28/30 combi it opens at 3 bar. Check your own boiler's manual, because other models and systems differ.
Can I replace a pressure relief valve myself?
No. It is a safety device on a gas boiler's heating circuit. Anyone employed to work on gas appliances in a home must be Gas Safe registered and competent in that area of gas work.
Sources
We cite the original source for every figure: government, regulators, standards bodies and manufacturers' own documents. Each one shows the date we last checked it.
- Worcester, Bosch Group. Greenstar 24/28/30 RSF combination boiler: Installation, commissioning and servicing instructions. Checked 11 November 2026. ↩
- Worcester, Bosch Group. Greenstar 24/28/30 RSF combination boiler: User instructions. Checked 11 November 2026. ↩
- Health and Safety Executive. Domestic gas: frequently asked questions. Checked 11 November 2026. ↩