After partially draining our system to remove a radiator, I refilled and bled the radiators but I've noticed that the pressure gauge keeps dropping to almost zero. I keep using the filling loop to bring the pressure up to 1 bar, but within an hour it will have dropped to 0.5 bar even with the boiler turned off.
I've checked the pipework I was working on and there are no leaks, I've checked the pressure valve on the expansion vessel and this gave out air only. I can see a bit of moisture around the slotted screws under the bulkhead(?) where the CH flow and return pipes join the main chassis, but not enough to even form a drip in an hour (with the boiler turned off and cold).
So after the small essay, my questions are:
Is this moisture likely to be enough to cause this much of a pressure drop?
Any idea what else the problem could be?
Could the pressure have been at virtually zero for while now, even though the CH and DHW seemed fine?
Our set up is a Worcester Bosch Greenstar 30 CDi system boiler with a hot water storage tank/emersion heater.
Thanks for any help, this is driving me nuts.