I had a new combi boiler installed a few months ago, converting my old open vented system to a sealed system. I didn't replace the old radiators. However, since doing that, I've noticed that the system seems to have air in it. I bleed the highest radiator about once a week and there's always a healthy hiss of air.

The pressure in the boiler is staying rock steady, so I'm ruling out a leak. My question is - does anyone have any idea where the air is coming from? The radiators aren't actually cold at the top so it's more an annoyance than anything else, but surely there shouldn't be any air in there to bleed, once I've already bled the system?

Background info: The new boiler is a Worcester Greenstar 30 Si. The system was flushed when the boiler was installed and had inhibitor added. The house (and, I guess, the radiators) is 25 years old.

Thanks!