Combi Boiler Losing Pressure — Diagnose and Fix It Yourself
A combi boiler that keeps losing pressure has a real fault — not a quirk. Repeated top-ups mask a leak, a failed PRV or a knackered expansion vessel. Here is how UK heating engineers diagnose it in the correct order.
Step-by-step method
1. Check the pressure gauge cold
Normal cold pressure is 1.0–1.5 bar. Under 0.5 bar the boiler will lockout. Over 2.5 bar hot means the PRV will dump water outside.
2. Inspect the PRV discharge pipe outside
Every combi has a copper pipe exiting the wall that terminates outside — the pressure relief valve (PRV) outlet. If it drips or shows limescale streaks, the PRV is passing and needs replacing (£15–40 part).
3. Bleed every radiator and re-check
Trapped air causes false-low readings once heat expands the water. Bleed all rads, top up to 1.2 bar, run heating for 30 minutes, then re-check cold.
4. Look for visible leaks
Check pipe joints under sinks, at radiator valves, and inside airing cupboards. Even a slow drip loses 0.3 bar a week. Damp patches on ceilings below bathrooms are the classic sign.
5. Test the expansion vessel
If pressure spikes hot and drops cold, the expansion vessel has lost its air charge. A Gas Safe engineer will re-pressurise or replace it. Cost £120–£220 fitted.
6. Check the filling loop is fully closed
A partially open filling loop lets mains pressure feed in continuously — the boiler will show high pressure then dump via the PRV. Make sure both taps on the loop are perpendicular to the pipe (closed).
Frequently asked questions
Is it safe to keep topping up a combi boiler?+
Only as a short-term fix. Fresh water introduces oxygen which corrodes radiators from the inside. Fix the underlying leak within a week.
How much does it cost to fix a boiler losing pressure?+
PRV swap £80–£150, expansion vessel repressurise £90–£140, expansion vessel replace £160–£240, hidden leak repair £150–£400 depending on access.