If your boiler or hot water cylinder is suddenly letting water out through the pipe outside, the pressure relief valve may be doing its job—or it may be signalling a fault. Many homeowners around Petersfield, Bordon, Liphook and Alton come across this issue at some point, and it’s often simpler to diagnose than it appears.
What the pressure relief valve does
The pressure relief valve (PRV) is a safety component designed to release excess pressure so your system stays within safe operating limits. When pressure rises beyond the valve’s set point, it opens briefly to discharge water. Occasional tiny releases during heating can be normal; constant dripping is not.
Quick checks you can do now
Before calling out an engineer, you can safely carry out a few simple checks. These won’t fix underlying issues, but they can help you understand what is happening.
- Look at your boiler’s pressure gauge when cold and when hot.
- Check whether your filling loop is fully closed.
- Observe how much water is dripping from the discharge pipe.
- Note whether pressure rises steadily while the heating is on.
Common reasons pressure relief valve leaks occur
There are several causes behind a PRV releasing water, and they vary between boiler systems and unvented cylinders used throughout homes in Whitehill, Farnham and the wider GU35, GU34 and GU9 postcodes.
1. The system pressure is too high
If the pressure climbs above the valve’s threshold—usually around 3 bar—the valve opens to protect the system. This may happen when the boiler is incorrectly topped up or when something is forcing pressure to rise abnormally.
Checklist for homeowners:
- Is the pressure above 2 bar when the system is cold?
- Does the pressure shoot up when the heating comes on?
- Has the system been recently topped up?
2. A stuck or faulty filling loop
A partly open filling loop allows small amounts of water into the system continuously. This gradually increases pressure until the PRV discharges. It’s one of the most common causes we see when visiting homes in and around Bordon.
Ensure both taps of the filling loop are fully closed. The handle position can be misleading, so turn them firmly into the closed position to be certain.
3. Expansion vessel issues
Your expansion vessel absorbs pressure increases when water heats and expands. If its internal air charge has dropped or the diaphragm has failed, the pressure has nowhere to go, forcing the PRV to open. Signs include sharp spikes in pressure when heating starts and rapid drops when heating stops.
4. A worn or damaged PRV
Once a PRV has discharged heavily, debris or limescale may prevent it sealing properly. This results in a constant drip even after the root cause is resolved. A faulty PRV generally needs replacing rather than repairing.
5. Heat exchanger faults
A pinhole in the boiler’s plate heat exchanger can allow mains water to seep into the sealed system. Because mains water pressure is higher, the system pressure gradually rises until the PRV releases water. This requires a Gas Safe engineer for diagnosis, but homeowners can often spot the pattern: a slow, steady rise in pressure even when the boiler is off.
6. Unvented cylinder pressure problems
If you have an unvented hot water cylinder, additional safety valves are present. The TPR (temperature and pressure relief valve) may discharge if either the pressure or the temperature climbs too high. Causes may include expansion vessel faults, blocked expansion pathways or issues with the inlet control group. These cylinders must only be worked on by qualified engineers with the correct certification.
How to reduce the risk of recurring leaks
Preventing pressure relief valve leaks is largely about routine maintenance and keeping an eye on system behaviour.
- Check your pressure gauge occasionally.
- Ensure your filling loop remains shut at all times.
- Book annual servicing to keep expansion vessels healthy.
- Act early—small drips often signal fixable issues.
When it’s time to call a professional
If the pressure rises sharply, the dripping becomes constant or you notice water staining near the discharge point, it’s wise to get the system inspected. Many homeowners in Petersfield and nearby towns such as Liphook and Farnham call once they see repeated discharges because the underlying fault rarely disappears on its own.
Understanding what’s happening helps you discuss the issue clearly with your engineer, but diagnosing and repairing deeper faults—especially anything involving gas appliances or unvented cylinders—must be left to trained professionals.
Next steps: If your pressure relief valve won’t stop dripping, book a professional assessment at https://www.embassygas.com/book