combi boiler not working

bing1955

Hi my combi boiler is dropping water pressure ,checked for leaks both upstair down stairs and seem fine isulated both up and down stair flow return pipes ,turned both valves of and put to 1.5 pressure and within about an hour drop to zero, tried vessel air pressure showing 1.0 bar.
works ok when heating on but when switched off drops right down.
could it be leaking on hot water side if so how do i check

macman

Make and model would help.
But the first thing to check is the PRV overflow pipe outside, if that is discharging then the valve is not seated properly, and needs replacing or cleaning.
If it's leaking on the CH circuit then it's either visible or it's not, no magic way to find it other than to access all the pipework. But start with the PRV, it takes 2 mins to check it.
It can't be on the DHW side with a combi, as the circuit is only 'open' with the hot tap open and calling for heat.