I really wish I knew, the idea is with on/off thermostat it is there to stop the boiler cycling in the summer, the TRV heads do the temperature control, but they can't turn boiler fully off, so without a wall thermostat it would cycle on/off all summer.
We are told the thermostat should be in a room normally kept cool that way it is turned off sooner when summer arrives, and in a lower floor room as heat raises, with no outside doors or alternative heating and no TRV in the room with wall thermostat, not sure on thinking with last bit. That sounds good, but what house has such a room? in the main we use the hall, and since it does have outside door, we also fit a TRV to get fast recovery when door opened yet slow completing reheat so rest of house can warm up.
But with an opentherm wall thermostat it would be fitted in the main room, there is the EPH thermostat which can be set as master and slave so up to 10 thermostats in the house which can link to motorised valves, but in the main it links to TRV's but would be placed in living room not hall.
So the whole way it works is different, so unless wall thermostat has been put in wrong place to start with, not a case of simply swapping, so likely looking at some wireless option.
So some one we assume who knows what they are doing fits an OpenTherm wall thermostat and also sets all the lock shield valves to correct settings, to replace a thermostat fitted likely by some one who does not know what they are doing, and failed to set any lock shield valves, so is the improvement because lock shield valves set correct, or because of OpenTherm control?
I fitted electronic TRV heads in my mothers house, and two wall thermostats in parallel one in hall and one in kitchen, wind and sun changed which end of house heated first, hence two thermostats, and will not pretend it took some time to tweak the lock shield valve settings, but it worked A1, did not have Opentherm option with her Worcester Bosch boiler, and the boiler seemed to modulate as designed to do.
Had to cheat with TRV heads, wanted 20°C at 8 am, so set to 22°C at 7 am and back to 20°C at 8 am as the anti hysteresis software in the TRV heads was OTT. Living room would still over heat with morning sun, but radiator cold so down to bay windows not heating system, after fitting the electronic TRV heads and setting lock shield valves it would some times hit 24°C but before I have walked into room sitting at 32°C as before setting the radiator got too hot too quickly then even when TRV closed it took too long to cool down again.
Except for sun on bay windows once set all rooms were spot on, this was without any ebus connection. Can't see how room control could have been improved. So only gain with OpenTherm would be less heat lost out of the flue. I am not convinced it would be that much.
If I had the option I would fit OpenTherm, my new house uses oil, so not an option. However although my thermostat will work with OpenTherm (Nest Gen 3) the main problem is it only measures the temperature in one room, in my case the hall, in USA Nest have released remote temperature sensors normally sold in lots of three, so it measures four rooms to ensure all are up to temperature, can't over heat as TRV stops that, but not released as yet in UK, I had thought Nest would connect to my Energenie MiHome TRV heads, but it seems when Google took over Nest support was removed.
So likely in my house Hive would work better as it does link to TRV heads, even if no OpenTherm, although Hive limited to 22°C. Both Drayton Wiser and Honeywell Evohome the OpenTherm is an extra module. Tado it seems does OpenTherm on one of their systems, but getting info on Tado is hard.
And using the phone to program TRV or wall thermostat is not very clever, as when I get home my mobile goes on charge, so I prefer the cheap eQ-3 heads which can be adjusted on the head to the far more expensive Energenie where I need a PC, tablet, or phone to change settings, so much easier just turning dial on eQ-3 until it says 20°C.
If the Nest remotes are released in UK I will get them, but as to getting another system and starting again, not worth it.