Boiler pressure keeps dropping — causes and fixes
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 suspect the pressure relief valve is discharging regularly (water dripping from a pipe outside the property or into a drain), the system pressure is too high — do not repressurise without first identifying the cause.
Not sure if this matches your problem?
Use our interactive tool — answer a few questions and get a personalised diagnosis.
Most likely cause & what to check
A sealed central heating system should hold its pressure between 1 and 1.5 bar when cold. A gradual drop over weeks is normal — a rapid drop within hours or days points to a leak or a faulty component.
Check every visible radiator valve, TRV body, and pipe fitting for damp. Even a slow weep from a compression fitting will cause significant pressure loss over days. Run your finger along pipework under floors if accessible.
Inspect the boiler itself — particularly the pump connections, the heat exchanger connections, and the auto air vent. A faulty auto air vent often weeps slowly and is missed.
Check the pressure relief valve (PRV) discharge pipe — usually a 15mm copper or plastic pipe that exits through the wall or into a drain. If it is damp or dripping, the PRV is opening. This means system pressure is spiking above 3 bar — the expansion vessel is likely waterlogged and needs recharging or replacement (£80–£150 fitted).
If no external leak is found, the heat exchanger inside the boiler may be cracking — allowing water to escape into the flue or combustion chamber. This is a serious fault. Call a Gas Safe engineer — a cracked heat exchanger usually means boiler replacement.
Once the actual leak is repaired, repressurise via the filling loop to 1–1.5 bar: attach the flexible braided hose between the two valves, open both slowly until the gauge reads correctly, then close and disconnect. If the system held steady after the last repair, monitor the gauge for two weeks.
Not confident doing this yourself?
Post the job and we'll match you with vetted local engineers. Free, no obligation.
🛠 Tools & materials you may need
Was this guide helpful?