I've got a combi boiler (Saunier Duval Thelia 23). The pressure in the
CH system is too low (about 1/2 bar, judging by the inaccurate gauge).
In the past (i.e. last year) I've refilled it via the filling loop with
no problems.

Now, however, I turn the filling valve and nothing happens. I can't
hear it filling, and the pressure doesn't increase. If I disconnect the
other end of the loop from the boiler and turn the valve, water comes
out at (what looks like) decent pressure, so I think the filling valve
(the one you turn) is okay. (This morning when I tried it, it *did*
fill for about ten seconds before it stopped.)

What's the next thing to look at? Is it likely that the one-way valve
at the other end of the fillin gloop is sticking, and if so what can I
do about it? (Would I need to drain the system and replace the valve?)

Thanks for any advice!

Robin