I don't have the option of OpenTherm, but back in 2019 when I moved into this house to find no way to control central heating without going outside down a set of steps, and back into the property under the main house and plug in the pump and switch on boiler, I selected a programmable thermostat which would control both CH and DHW with 2 wires, and with an extra low voltage on those 2 wires, as colours changed and the third wire was open circuit.
I selected Nest Gen 3, which I was told would work with the 4 Energenie MiHome TRV heads I already had, it did the control with 2 wires, but failed to work with the TRV heads, and has proved to be inadequate to control the main house.
The big problem is there is no one good place to measure the temperature, I am using the hall, but every middle floor room, (lowest floor for main house) has either doors to outside, or alternative heating, be it cooking, wood burning fire, or the sun through windows, it needs more than one sensing unit, Nest USA do a sensing unit
but not released in UK yet, and Nest USA is very different to UK version. In the main in the UK we use a sensor built into the TRV, Hive, Drayton Wiser, Honeywell EvoHome, Tado, and others all use the TRV as the remote sensor, I do use electronic TRV heads,
the cheap one actually works better for me, no need to find my phone to change the temperature, one button toggles between the pre-set by me comfort and Eco temperatures, or the dial can be pressed for boost or turned for new setting, where the much more expensive version
I need to use phone or PC to set, well if you trust Nest Mini you could use voice commands, but they seem to act like mischievous little boys, not sure I would trust them with central heating.
At the moment the heating is not doing very well
I really want the rooms warmer, but note hall,
Nest Gen 3 is showing 2ºC higher than the TRV but in the same room, so to ensure the wall thermostat stays on in cold weather, the TRV has to be set lower than the wall thermostat, it was cold this morning, -5ºC showing for outside.
Although it says 15ºC for living room, baby alarm on floor shows 13ºC and the room does not actually feel that cold, more like 18ºC as the sun is warming it well. I have never yet lit a fire in the grate, maybe I will this year. But the location of any thermostat is important, you want it in the natural air flow, and in most rooms the TRV is in the best location to work out temperature of the room, mounted on walls it measures the wall temperature as well as air.
