Boiler pressure keeps dropping — why and how to fix it
Check the steps below first — if you're not confident, get it fixed safely today.
Post a job — we'll find you an engineer →⚠️ If you need to repressurise more than once a month, there is a leak or a fault — investigate before continuing to add pressure. Repeatedly topping up a leaking system can introduce oxygen and corrosion inhibitor imbalance.
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Most likely cause & what to check
A sealed central heating system is a closed loop — it should not need topping up more than once or twice a year under normal circumstances. If the pressure drops repeatedly (monthly or more), there is either a leak or a failing component.
Check the pressure relief valve (PRV) — a brass valve on the boiler pipework with a discharge pipe running to outside or into a tundish. If this is weeping or discharging, the system pressure is exceeding 3 bar somewhere (possibly expansion vessel failure) and the PRV is releasing it. Feel or look for dampness at the valve or its discharge pipe.
Check the expansion vessel — all sealed systems have a pressurised expansion vessel (a red or grey steel sphere, usually in the airing cupboard or attached to the boiler). Over time the internal diaphragm can fail, reducing its capacity to absorb pressure changes. A Gas Safe engineer can test it with a tyre pressure gauge on the Schrader valve on the top/side of the vessel. If the pressure is zero (should be ~0.75–1 bar), the diaphragm has failed.
Inspect all visible pipework, radiator valves, and connections for drips or damp patches. Pay attention to pipe joints behind radiators, under floorboards if accessible, and at the boiler connections. Even a slow weep losing a few millilitres per day will cause monthly pressure drops.
Check the filling loop itself — if either isolation valve on the filling loop is partially open, mains water continuously enters the system and raises pressure until the PRV discharges. Locate both valves and confirm they are fully closed (handle perpendicular to the pipe).
If no leak is visible and the expansion vessel is serviceable, a Gas Safe engineer should inspect the boiler heat exchanger and all internal seals. Ongoing pressure loss without a visible leak often points to a pinhole in the heat exchanger.
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