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!

It means you have a leak somewhere. Even with a pressurised
system, air will leak in through leaks. Such leaks can be hard
to find as the moisture can dry on hot pipework before it gets
noticed.

If the leak is slow, I suggest waiting until you switch the
heating off for summer, and then use the filling loop to increase
the cold pressure to match what the highest hot pressure is.
Then go round looking for leaks which will hopefully become
more obvious. Also bear in mind the leak uses up your
inhibitor more quickly than normal and you might want to
schedule replenishing that at the same time, when you've
fixed the leak.

One other remote possibility -- what is your cold pressure, and
how high above the pressure guage are the highest heating pipes
or radiators? If you have the guage in the basement of a 3 or
more story house such that there's more than 30' head of water
above it, you might need to increase the cold pressure of the
system to ensure it's all pressurised all the time.

--
Andrew Gabriel