Take the pressure out of the boiler when it cold, take it down to just over 1bar. Then find the stop cock for the house and turn it off, now run the boiler on CH and see what happens to the pressure? The pressure should rise to no more than 2 bar when hot, and will drop back to 1 bar when cold. If this is the case and the pressure stays at these levels with the water to the house switched off, then mains water must have been leaking through the filling loop or the plate heat exchanger.
Justin, March 2008