I thought of sharing my solution to the title; please note that this is valid for my configuration which is Vitodens 200-W, underfloor heating, weather controlled, low loss header (Q70 type) and mixer with separate pump for the heating circuit and that I am not using 100% Viessmann accessories. I am using EM-EA1, mixer and EM-M1, but commands are given by other accessories (room thermostats for the underfloor heating).
It may not work for you (depending on specificities of your systems) but it may provide some food for thoughts!
Under the initial configuration of the system (same as above less low loss header, mixer and EM-M1, but mixing units on each floor of the house) the on/off command to the boiler (depending on the heating requests from each room, all rooms are zoned and independently controlled) worked OK – in the sense that the boiler and internal pump turned on only when a heating command was sent to the boiler by the room thermostat and turned off when the target temperature in the room was achieved.
The commands are achieved via the D1-D3 connectors in the EM-EA1 and are sent by the room thermostats (but you can use a DIY circuit) and with the boiler set up like this: system on, heating mode off and DHW on.
However, whenever the outside temperature fell under a certain value the internal pump of the boiler turned on, irrespective of whether a heating request was active (antifreeze function, the pump ran through the internal boiler bypass). I have been told by Viessmann that for my boiler there is no way to make any changes to the antifreeze function … ☹
Further on I Installed a low loss header, mixer and EM-M1 (and got rid of the floor mixing units). I am using the same settings of EM-EA1 and the boiler (heating off, DHW on – time schedule) and now I haven’t seen the antifreeze kick in (same temperatures as before, and even if it does, the boiler pump runs through the low loss header) and when the target temperatures are met the heating circuit pump is off, and so is the boiler pump, irrespective of the outside temperature.
Under the above layout I have seen no issue with the antifreeze function and pumps running all the time – everything is controlled by the room thermostats. I note that I have no options in the menu for choosing anything for the internal pump (apart from low, max and set running percentages, some useless, as in weather controlled with low loss header the internal pump run at constant speed). I am thinking that same effects can be achieved for radiators as well, not only underfloor heating, depending on the controls for the radiators and the interface with EM-EA1.
Hope you will find this interesting/helpful and all the best!
