why does my combi boiler loose pressure

[kirstynurse]

have had a plumber in several times and he has fixed valves and things on rads to ensure they arent leaking. he has replaced the automatic air vent as the washer had gone on that but boiler is still loosing pressure it seems fine whilst the heating is on and then once the heating is off the pressure drops form 1 to nearly 0 overnight. could the pressure gauage be faulty

 

[copperspropper]

sounds like the pressure vessel needs recharging or renewing

 

[Grumpygrouter]

Had same problem with my Worcester HiFlo. New expansion vessel cured it.