hello guys . went around to boiler. worcester combi 25 cdi greenstar. noticed pressure was at 2.5 bar. assuming installer had overcharged it went to let some water out of rad and all that came out was air, lots of it. anyway i then noticed the gauge dropped down to 1 bar. just want to know can a system which hasnt been aired cause rise in system pressure gauge and if so can someone explain? many thanks