Boiler & Appliances RU

Boiler Pressure Keeps Dropping: Causes and Home Fixes

~7 min 5 steps Verified:

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

Step-by-step fix

  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. 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 — FAQ

Frequently asked questions

01How 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.
02Is a failed expansion vessel dangerous?
Not directly, but pressure will repeatedly hit 3 bar and vent water, causing corrosion and damage. Repair it promptly.
03Why 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.