I have a Viessman Vitodens 100 combi boiler which I can only get hot water out of. Some times when I keep resetting the boiler I can get heating but once hot water is called for it won't divert back to heating once the tap is closed. The tap sign is only visible even though the heating is only being called for. Any ideas, I suspect the diverter valve to be the problem but I'm not entirely sure.