Honeymonster wrote:

My open vented central heating system suffered from air ingress so I
had it converted to a sealed system. However, this has not cured the
problem, air still collects in one of the upstairs radiators. I've
done the hydrogen test and it really is air. Any idea how it can be
getting in? The at cold system pressure is constant, so I presume
there are no leaks. As the air collects the pressure doesn't rise but
when I bleed it out I then have to replenish the system via the filling
loop to restore the normal "at cold" pressure. It makes no sense to
me!

Are you sure that it is air? If you do not have enough inhibitor you
could be getting a reaction that is releasing hydrogen and this would
have the same effect in the radiators.

I don't know how safe it would be to test with a match as you bleed a
radiator.

Andrew