Qe don't have a combi boiler. It is an open system. Shall I listen for the header tank in the loft filling once the heating is on? Shall I try and make him hang around for a while 9nce it is all on?Your boiler should have a pressure gauge, it'll rise and fall a bit when heating turns on and when the system cools. Just keep an eye on it to see if it falls alot more than through normal use.