Hi, I’m having a problem with hw venting into the small header tank. This happens when it’s set to hw only or CH. I’ve tried lowering the water level in the header tank but it makes no difference. The incoming hot water is being taken back into system via the primary circuit 15mm supply so fortunately the level of water in the tank is remaining stable at whatever the level is set at. The system to all intents and purposes seems to be working, supplying HW and heating the radiators but it is unnerving having a tank of quite hot water sitting in the loft. My service engineer put MC3 Cleaner in the header tank about a week go but this has had no effect on the venting. Any helpful suggestions would be gratefully appreciated. Baz.