I've just had fitted a Worcester combi boiler to replace an existing combi
and is a sealed system.

The problem I've had is that the upstairs radiators constantly fill with air
that I purge only to reappear after a few days. This is causing the pressure
to increase in the system each time.

Can't find any leaks and I don't need to top up the water. Is there any type
of valve in the boiler that may let air in but not let water out?

Mezza