Opentherm control and mixing valve

[birrmann]

Hi All,

Can you offer some advice please regarding the intended Evohome setup?
I am planning to upgrade to evohome and would love to use the opentherm control - new Ideal Vogue boiler fitted in next few days.
House is set up in 2 zones; downstairs is all UFH and upstairs all radiators (5 bedrooms).
I would like to install evohome, that for now would control just the above 2 zones, I could add in more controls for individual bedrooms in the future.
So here's my plan:

Boiler controls:
1x Evohome wifi connected controller
1x opentherm gateway R8810

Downstairs zone controls:
1x HM80 mixing valve controller with flow temperature sensor
1x mixing valve actuator (Siemens SS31A should work with HM80? Is there a honeywell equivalent?)
above mentioned main evohome controller to act as thermostat for this zone

Upstairs zone controls:
1x BDR91relay to open/close zone valve
1x HCF82 room sensor

Can you advise if this set up will work, but also how evohome will control boiler's flow temp while adjusting the mixing valve position to meet downstair's setpoint?
Scenario 1:
If we can assume that upstairs zone (rads) is not calling for heat - will evohome modulate the boiler down as low as required to meet temperature that mixing valve is trying to achieve for underfloor heating? or will it just fire the boiler up at some unknown flow temperature and let the mixing valve controller do its job and mix it down as required? (in simple terms will evohome modulate the boiler down to same/similar temperature that mixing valve "wants" to achive?).
Scenario 2:
both zones calling for heat - I assume that rads will be at higher flow temperature so evohome will tune the boiler's flow as required to meet upstairs demand and at the same time mixing valve will do its job and mix the water to desired temperature?

Some notes: downstairs is all open plan so I do not want to use the UFH controller, all I want to do is to control the UFH flow temperature via the HM80 if possible. Regarding upstairs at this point in time I can't afford to install HR924s on each individual rad, so have to set up/control all of upstairs using existing zone valve and single temp sensor.

Your help will be much appreciated.
Regards,
birrmann

[Richard]

Give me a call or email to discuss this properly!