You may have left the filling loop open when you have filled it up in the past. When you pressurise the system always fill the system when cold other wise you can damage the heat exchanger.(PRESSURISE TO 1 BAR)
Also if the pressure gauge rises over 3 bar after you have correctly filled the system and water is expelled from the pressure relief there could be a problem with the expansion vessel.The expansion vessel may have a damaged diaphragm or may have just lost charge which should be around .75 to 1 bar.This should be checked by a corgi engineer.
Russ Davies, February 2009
