Boiler Pressure Too High — Causes and Fixes

What Is Normal Boiler Pressure?

For most combi and sealed system boilers, the normal operating pressure is:

When pressure rises above 2.5–3 bar, it indicates a problem. The pressure relief valve (PRV) is designed to release pressure as a safety measure if it gets dangerously high — so you may notice water dripping from a pipe outside your property.

Common Causes of High Boiler Pressure

Overfilled System

The most frequent cause is over-pressurisation during the last fill. If someone recently topped up the pressure and added too much, the gauge will read too high.

Fix: Bleed one or more radiators to slowly release water and reduce the pressure until it reads around 1.2–1.5 bar cold.

Faulty Expansion Vessel

The expansion vessel is a small tank within the boiler containing pressurised air. It absorbs the expansion of water as it heats up. If the expansion vessel fails (either by losing its air charge or the membrane failing), water pressure rises sharply when the boiler heats up.

Fix: A Gas Safe engineer can recharge or replace the expansion vessel — a common and reasonably straightforward repair.

Stuck Filling Loop

If the filling loop valve is stuck slightly open, it allows mains-pressure water to constantly enter the system, gradually increasing the pressure.

Fix: Check that both filling loop valves are fully closed. If pressure continues to rise even with the loop closed, the internal valve may be leaking and needs replacing.

Faulty Pressure Relief Valve

If the PRV is stuck open, it will continuously release water, and if stuck closed, pressure can build dangerously without a safety release. A dripping pipe outside is usually a sign the PRV has activated.

Fix: Do not attempt to work on the PRV yourself — call a Gas Safe engineer.

What to Do Right Now

If your boiler pressure is above 3 bar:

1. Turn off the boiler

2. Do not use hot water or heating

3. Call a Gas Safe engineer

Halo Heating can diagnose and fix high pressure issues quickly. Call 01483 676108 — we're available 24/7 for emergencies.