Boiler pressure keeps dropping after topping up
Check the steps below first — if you're not confident, get it fixed safely today.
Post a job — we'll find you an engineer →Topping up your boiler pressure is a quick fix — but if the pressure keeps dropping back down within days or weeks, something is causing the loss. This guide helps you identify the fault before calling an engineer, so you can give them the right information and avoid unnecessary call-outs.
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
Check all visible radiator valves, pipe connections, and boiler joints for any sign of drips or damp. Even a slow weep can cause the pressure to drop over time.
Check the pressure relief valve discharge pipe — this is usually a 15mm copper or plastic pipe that exits through the outside wall. If it is dripping or damp at the end, the PRV is releasing pressure. This means either the system pressure is being set too high, or the PRV itself has failed.
Check the filling loop — the braided hose or rigid connection under the boiler used to repressurise. If the valves on the filling loop are not fully closed, water slowly enters the system and then bleeds back out via the PRV. Both valves should be closed after every top-up.
If there are no visible leaks: the expansion vessel inside the boiler may have lost its charge. This is a common cause of repeated pressure loss and requires an engineer to test and recharge or replace it.
Unlock the full step-by-step guide
This guide has more steps written by a qualified plumbing and heating engineer — subscribe to see them all and save yourself a costly call-out.
Frequently asked questions
How often should I need to top up boiler pressure?
You should almost never need to top up pressure in a well-maintained closed system. Needing to do it once a year might be acceptable; once a month or more is definitely a fault.
What is an expansion vessel and why does it affect pressure?
The expansion vessel absorbs the increased volume of water as it heats up. If the vessel loses its air charge, it can't absorb the expansion, causing pressure to spike and the PRV to release water — leading to repeated pressure loss.
How much does it cost to recharge or replace an expansion vessel?
Recharging costs around £50–80 in labour. Replacement of the vessel (if failed beyond recharging) typically costs £150–300 including parts and labour.