Sounds like it could be the automatic air vent that releases air out of the system. When you top up a system and it then heats up dissolved oxygen is then driven off this is then released by the vent (dropping your system presure )when a system is first filled up. This can go on for a few weeks but will then setle down when all the oxgen is driven off.Other than that try the ferroli helpline (technical option )for installers they are very helpfull. Hope this is of some use
