I had same with Mothers old house, the latter Greenstar boilers could use a Bosch thermostat connected to ebus, but hers was a simple on/off, I decided to fit programmable TRV heads, with the intention of fitting a Nest wall thermostat latter, which the spec said would work with Nest. However the TRV heads cured all my problems in that house, so never did up-grade to Nest, new owners did not want the Energenie MiHome TRV heads so swapped back to non programmable and brought them here to new house.
Central heating in this house was a mess, so Nest Gen 3 seemed to be a good option, mainly as can connect the thermostat to heat link with two wires allowing CH and DHW to be controlled with thermostat, and keeping battery charged all with two wires, and that was all I had between main house and where the boiler was, so seemed ideal solution.
However it seems when Google took over Nest they removed support for Energenie TRV heads, in USA they released temperature sensors so it could monitor 4 rooms, but not released in UK, so Nest Gen 3 is a white elephant, it can only monitor the temperature in one room, if the boiler is OpenTherm enabled (i.e. not Bosch) then it does have some advantages, but in the main even Hive is better than Nest, the EvoHome or Drayton Wiser are far far better with an on/off boiler.
Since I now use oil the boiler I now have does not modulate (turn down) but with a modulating boiler the wall thermostat does very little, the TRV heads do all the work, but the TRV head can't turn the boiler on, so we still need a wall thermostat to turn on boiler in cool weather and off in warm weather, a simple bi-metal thermostat could do that.
As to geofencing, Nest does not allow you to set distance, you set the Eco and Comfort setting, and trust Nest gets it right, because there is a PIR built into the Nest wall thermostat once you walk past it then it shows comfort setting, so you think it is working, however when the local EE mast was taken out in storms earlier this year, I had a cold couple of days until I realised Nest was turning heating down to Eco all the time as did not know my phone was at home, and I had to disable the geofencing.
Other systems it seems you can set the distance at which the heating comes on.
As said with mothers fitted the smart TRV heads first, and there was no need for a smart wall thermostat, well the Energenie I fitted were not that smart, Wiser work out how long it takes to warm the room and start heating before set time so at temperature at set time, I had to set Energenie 2 degs higher for an hour to over ride the OTT anti-hysteresis software built in.
I hope Nest release the temperature sensors in the UK, as without them it is rather useless.
