A bit short on info here but it sounds like your pressure vessel doesn't have enough air in it. This PV has to be pressurised by pumping it up so that as the water in the system heats and expands the air is squashed and the pressure stays within limits. Your system sounds like the PV has insufficient air in it. You can normally pump these up with a hand pump but you need to do it when there is no pressure on the water side. Once you've pressurised the PV correctly then you can use the filling loop to fill the system to its normal cold pressure. Hope that helps!
Richard, December 2008
