Do you actually have the H1 extender module. If not, it's my understanding that the viessmann only responds to its own clock and the external weather sensor. From your wiring it appears that the thermostats merely switch the zone valves open or closed. The viessmann gives priority to DHW so if the DHW sensor isn't satisfied it will only 'bleed' a reduced amount of flow to the CH circuits. Once DHW is satisfied full flow goes to the CH circuits.
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