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Hi there. Thanks for your reply. Very informative. We are not using Open Therm but the on/off system. The diagram that best looks like what we have I would say is diagram 4 (fig.13) but with no wire connected to number 1 terminal on the right hand side of the diagram (marked ‘D’) I am assuming...

Hi all. First time posting on the forum so I’m hoping someone can point me in the right direction with regard to wiring a new boiler thermostat? I have a Viessmann Vitodens 100-w combination boiler currently connected to an intermittently working Honeywell CM921 wireless thermostat. At some...