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The unvented hot water gas boiler system is loosing pressure overnight. so gas engineer checked PRV, pressure vessel and gas boiler for leak.
no leak found. but the pressure goes to 0 at night when system is not active and morning at 4 am boiler is scheduled to start, which makes gurgling noise and cuts off. so i top up pressure and boiler works.

i can't see any leak on the floor and ceiling.
could it be a leak in unvented cylinder coil inside so during night it allows water to leak but during day pressure stays 1 bar.

Need to get leak expert to check the floor / underground for leak. but need suggestion.

If the expansion tank is damaged or not big enough, when the water heats up and expands a relief valve will open dumping the excess water to reduce the pressure, then when it cools the pressure will drop.

The expansion tank is normally in two, with a diaphragm between the two, one side should have water, the other side a gas, there is often a tyre valve on the gas filled side.

If the diaphragm fails, you end up with water both sides. I have in the past blown it up again with bike pump, but don't think it should be done this way, but I am an electrician not a plumber.