I've moved into a house with a Vaillant Thermo-Compact combi boiler. The
pressure guage has been reading 1bar which is OK according to the
installation manual I've found. However, one of the radiators had a
large amount of air in it, and after bleeding it, the pressure has gone
down to about 0.5bar. So I looked for a filling loop, and discovered it
isn't meant to have one, it's meant to be a sealed system with a
mechanism for venting air built into the boiler. There are diagrams in
the install manual showing that it fill when commissioned by connecting
the CH return to the mains temporarily. So I looked around for a
connection to do this and found that in fact there is a pipe coming from
what looks like the mains into the CH return. This has been permanently
plumbed in with two stopcocks right next to each other. So I presumed
this would be the way it fills. But, when I open the valves, nothing
happens. The pressure does not go up, there's no sound of wather flowing.

At this point I admit defeat. The heating still works, but I'm worried.
Any advice?

Thanks

John