Boiler Pressure Keeps Dropping: Causes and Home Fixes
Independent guide — no official affiliation with the platforms mentioned.
Short answer
If you refill every 1–2 weeks, something is losing water: the usual suspects are weeping radiator bleed valves, a not-fully-closed filling loop, silent discharge through the safety valve or a failed expansion vessel. Check the loop and valve first; if pressure rises while heating then falls, the expansion vessel must be tested.
01 — Step-by-step fix
Step-by-step fix
ADIM 01
Make sure the filling loop is fully closed
A half-closed or worn loop lets water through silently, pushing pressure up and opening the safety valve. Close it fully and wrap it with a dry tissue for 1–2 days: a wet tissue means the loop must be replaced.
ADIM 02
Check the safety valve and discharge pipe
Drips or moisture marks on the copper discharge pipe under the boiler mean the system vents through the valve — often a sign of expansion vessel failure or overpressure.
ADIM 03
Inspect bleed valves and joints
Look for dark water marks or damp under bleed screws, radiator unions and the pipework under the boiler. Small leaks evaporate while silently dropping pressure.
ADIM 04
Test the expansion vessel
Note the cold pressure, then run heating: a rise above 1 bar that shoots to 3 bar and opens the valve means the vessel has lost its air charge. The Schrader valve under its plastic cap can be checked/recharged (0.8–1 bar); a failed membrane needs service.
ADIM 05
No visible leak? Book a leak test
Underfloor/wall pipework hides leaks. Thermal camera, pressure and tracer tests locate the point and avoid needless wall demolition.
02 — Frequently asked questions
Frequently asked questions
How much pressure drop is normal?
A one-off 0.2–0.3 bar drop after initial bleeding is normal. Needing a refill weekly means there's a fault.
Is a failed expansion vessel dangerous?
Not directly, but pressure will repeatedly hit 3 bar and vent water, causing corrosion and damage. Repair it promptly.
Why does underfloor heating drop pressure more often?
Long hidden pipe runs and manifold connections are frequent leak points; also check the manifold valves are tight.
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