I've been having a problem with air in a sealed system, not huge
amounts but enough to make the system noisy. It only seems to be
noticable when the hot water is on by itself, and i'm not getting much
air in the rads.

- Definately air not hydrogen, and there is plenty of inhibitor in
system.
- Quite a large system, 30KW condensing boiler, 6m head Grundfos Alpha
pump set to max. Pressure vessel is between boiler & pump intake.
- Zone valves are on returns, not feeds.
- Boiler, pump, zone valves etc all downstairs, and there is quite a
long run to HW cylinder (17m of pipework approx).
- System pressure seems to remain constant, and does not drop over
time.

I've pressurised system to 3bar and checked for leaking compression
joints, rad valves etc, everything seems fine.

How long roughly does it normally take for air in the system to clear
after a refill on a sealed system? (note there is no air separator)

I'm pretty sure air is being drawn in, any suggestions to what the most
likely causes could be?