This would indicate the system water pressure is low. It needs topping up to 1-1.5bar when the sytem is cold.
Unfortunately for you, the silly buggers at Volkera uniquely decided to hide the pressure guage inside the boiler. So removal of the front case is necessary to set correctly.
There are two screws hidden underneath the front cover, you need to undo these to get the cover off, then open the filling loop until the guage reads as above and reset the boiler.
Carefree
July 2010