Filling loop left open
The most common cause. If the silver braided hose valves under the boiler weren't fully closed after topping up, mains pressure keeps forcing water in. Close both valves and bleed a downstairs radiator to drop pressure to 1.2 bar cold.
Failed expansion vessel
The vessel absorbs water expansion as the system heats. Once the internal bladder ruptures, pressure spikes sharply when hot and often triggers the PRV to discharge outside. Recharging (short-term) or replacement (proper fix) needed.
How to reduce pressure safely
1. Check the filling loop is closed.
2. Attach a bleed key to a downstairs radiator with a cloth held below.
3. Open the bleed valve slowly — water will trickle out.
4. Watch the pressure gauge and close the valve at 1.2 bar cold.
When to call an engineer
If pressure keeps climbing after bleeding, or the PRV pipe outside is dripping, book an engineer — the expansion vessel or PRV needs work. We cover every KT/TW postcode.
Pressure keeps climbing? Book an expansion vessel check across KT/TW.
Call 07932 898534Frequently asked questions
Is high boiler pressure dangerous?
Not immediately — the PRV vents excess pressure outside. But ignoring it stresses seals and joints, and can eventually cause leaks inside the boiler.
Can I bleed the radiator whilst the boiler is on?
Turn the heating off first. Live systems spray hot water and won't lose pressure smoothly.
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