To me it looks like the thermostat is a simple switch that completes the circuit when it drops below a temperature. Or am I being too simple?
Yes I have a few times wondered with a modern gas combi-boiler why we need a wall thermostat, and on/off switch with wifi could do the same job.
However it is down to what you can link to with the wifi. IFTTT can in some cases do what you want, but what happens if wifi goes down?
However there are two very different thermostats, there are OpenTherm and other similar methods which slowly turn the boiler output up and down, and there are on/off basic designed to turn off the boiler when warm weather arrives to stop the boiler cycling.
Location is important, to stop boiler cycling we place the on/off wall thermostat where it will remain warm over night and in a room normally kept cool, of course with no outside door, alternative heating and normally on ground floor, on a spring morning when likely to be a fine day we don't want it to fire up the boiler when latter in the day the house will likely get too warm, although hall does have outside door, it is often used for the thermostat as we keep halls cool, so can be set to 17°C and still allow central heating to run, so more likely to turn off boiler in warm weather.
But with the modulating thermostat it is likely placed in the main room.
There seems to be a problem getting people to use the TRV's to control rooms, and after living in my late mother's house with a modern modulating boiler I can understand why, I found it hard enough changing from °F to °C but *123456 marked on the TRV was about as much use as chocolate fire guard. So we had a lock shield valve on supply, and TRV on return which seems wrong way around, and some how your suppose to work out how to set them. Once a had a TRV marked in °C then I could set them, my head had current and target temperatures shown, so current above target close lock shield a bit and current below target open lock shield a bit and once set the TRV was spot on. And swapping back to *123456 I found 3¼ gave around 20°C and still worked spot on once the lock shield had be set using the one with °C marked on it.
However the whole idea of the TRV sending a wifi signal to a wall thermostat/switch to control boiler failed as I simply could not get them to talk to each other. Nest Gen 3 wall thermostat should have talked to the Energenie TRV but it was hit and miss. Since both are thermostats I simply set to same schedule so temperatures matched, the TRV was IFTTT, but I have never really got my head around IFTTT to control them.
So for wifi to work I would say wants the be the same manufacturer, so Hive wall thermostat and Hive TRV heads at least for important rooms likely will work well, but unless you can find a TRV to work with the wall switch then there may be problems.
I have only used the geofencing with Nest wall thermostat, although geofencing can be used with Energenie I have found once the Nest is turned down, the radiators don't heat up anyway, so no need for both to use geofencing, obvious really. It would depend how long the heating is turned down for, if for days then geofencing for TRV may make sense, but I think unlikely.
As to manually changing temperature from the phone, only time I do that is for cooling, I will look at room temperature reported on the TRV to decide if to turn AC on, but that is only time I manually use phone.
So for a 10 hour gap likely any geofencing switch would work, as you say all you want it to do is switch off heating when you leave, and switch it back on before you arrive home again, so any switch, maybe with a frost stat across contacts in case really cold would do the job.
