I've just had a completely new system installed. The boiler is a BAXI
'System' 35/60 with pump and expansion vessel built in. We have 6
Radiators plus indirect cyclinder and a 3 way valve.

My question is this. As long as I leave the system running, ie HW
and/or CH demand on 'all day' the pressure sits between 1 and 1.5 bar
all week, or for as long as you keep it all running.

If I let the system go off, ie do not programme CH or HW to come on at
all (say over night), and allow everything to cool right down the
pressure drops right down to almost 0 by the morning.

I have been all over the system and cannot find any leaks and it only
takes a small amount of water through the filling loop to bring the
pressure back up.

My question is this. Why does the pressure only drop when the system is
allowed to cool right down? and could it be related to this... In the
commissioning stage of the install instructions for the boiler it says
to open the screw on the automatic air valve in the boiler. It does not
mention closing it again. I have checked and the screw is definately
loose (open). Is this correct? Or should it be closed once the system
is filled and bled?