Adding wet UFH to my existing Drayton Wiser central heating system

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As part of an house extension, I am adding wet UFH to my existing central heating system. 

Current hardware:

• Viessmann Vitodens 100-W BLKF
• Drayton Wiser HubR
• Multiple wall and radiator thermostats
• Wago mBox Wiring Centre

 

System is working great so keen to build on it rather than replace with something different.

 

The main issue is that the Wiser HubR is connected to the boiler via Opentherm and according to Drayton this is not advised.
 

Quote

In regards to using the OpenTherm, when there are multiple heating circuits on a combination boiler, it will not be possible for both circuits to utilise the OpenTherm protocol at the same time. If one of the circuits was experiencing demand at a low level it would request a lower flow temperature from the boiler, however if the 2nd circuit then requested full demand and therefore a higher flow temperature, the boiler would prioritise the higher demand.
 
This would result in the 1st channel receiving more heat than expected and protocol would not be completed efficiently followed.

My plumber kindly left the cables for the Floating Switch contact connection in the wiring centre ready to make the switch so no need to open the boiler.

Future hardware:

• Viessmann Vitodens 100-W BLKF
• Drayton Wiser HubR
• Multiple wall and radiator thermostats
• Wago mBox Wiring Centre
• Drayton Wiser Underfloor Heating Controller
• Mid position 3 port valve (CH,UFH)
• Finder 22.21.8.230.4000 Monostable Modular Relay SPST-NO 230VAC 20A (boiler expects  

 

Could you please sanity check the wiring diagram below

 

 

 

Reasoning Behind the Relay is that although the Hub-R and Wiser UFH Controller outputs are “voltage-free capable”, they must be linked from terminal L to provide 230 V. Once linked, they are no longer volt-free.
To avoid sending 230 V into Viessmann Port 96, a relay isolates boiler demand completely. This is the part I am questioning myself over. Would be good to get a second opinion