Boiler keeps losing pressure and switching off
Check the steps below first — if you're not confident, get it fixed safely today.
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Most likely cause & what to check
Top up the pressure to 1.2–1.5 bar using the filling loop to get the heating working temporarily.
Check outside the property for a small pipe (15mm copper or plastic) dripping water near the boiler flue — this is the pressure relief valve (PRV) discharge pipe. If it is wet or dripping, the PRV is opening and releasing pressure.
Inspect all radiator valves, pipe joints, and the boiler connections for any signs of dampness or drips. Even a slow weep will cause repeated pressure loss.
Check the filling loop valves are fully closed after topping up — a partially open filling loop lets water trickle in and then be expelled by the PRV.
If no leaks are visible, the expansion vessel inside the boiler has likely lost its pre-charge pressure. Call a Gas Safe engineer — they will test the vessel with a gauge and either recharge it with nitrogen or replace it.
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Frequently asked questions
Can I keep topping up the pressure and ignore the fault?
You can keep the heating running this way short-term, but repeatedly opening the filling loop adds dissolved oxygen to the system which accelerates corrosion and sludge build-up. Get the underlying fault fixed.