Nest e is battery operated, Nest Gen 3 is two units which can either be hard wired together or the wall thermostat can be USB powered, there is of course nothing to stop you mounting the Heat link and wall thermostat next to each other.
The wall thermostat needs to be where the natural circulation will warm it with the room, normally wall opposite to radiator, EvoHome is not really a wall thermostat it is more of a hub, and it reads the temperature from the TRV head so each room is individually controlled and the wall thermostat adds up the requirement for each room and tells the boiler either what output is required when using OpenTherm or when to turn off/on if not using OpenTherm, I think Todo is same idea.
Nest tells the TRV head what temperature is set so wall thermostat and TRV's are all singing from same song sheet, but it does not read the temperature of each room. It uses Energenie TRV heads. Nest can also connect to boiler with OpenTherm.
Hive does not have OpenTherm and it does not work like the rest, the TRV head send a heat on demand signal to the wall thermostat which means it switches on for 1/2 hour each time it gets a signal so the boiler modulates due to return hot water temperature.
However all four it's the TRV head which controls the room the wall thermostat is just what connects the whole system together with Nest being now bottom of the pile, don't like admitting this as I have Nest.
So with any old wall thermostat you can still control the home simply by altering the TRV head with phone/tablet/PC, at the wall thermostat does is stop the boiler cycling in the summer, so no wiring required, just replace the TRV heads, so you get something like this
on your phone, the one that says "You are home" is the Nest wall thermostat ignore flat heating that is a socket, two radiators in living room that's why two you can see current in red and target in blue clicking on them allows you to set that radiator, and so that room,
thumb nail shows what it shows when you select, Nest wall thermostat can either be set with Nest software or the Energenie software. Yes with Nest I can change one which changes all, but without Nest I could still control each room anywhere in world where the phone will work so Nest is only really needed to turn on the DHW when I decide I want a bath when getting home.
The problem is TRV heads are £40 each working with wifi, so in bedrooms I just have bluetooth TRV heads at £15 each.
However the point is a simple wifi on/off switch can turn whole heating on/off, no need for a wifi thermostat, and I can't see why I need to change temperature, all I really need is two thermostats, one stops it freezing the other stops the heating from cycling. The reason I have Nest is there are only two wires from flat where boiler is to house and I can control the pumps, motorised valves and boiler with just those two wires with the wall thermostat, I did not get it because of the wifi.
I will admit I have used the phone to control the heating this summer, I would look at the current temperature in the room, and if too hot I would remotely switch on a socket into which the AC was plugged so cool down the room. Mainly as AC is bit on small size and it takes ages to cool room. Central heating is in the main timed, and if I get home early it does not take long to heat up the room, and I don't use whole house when I get home, I just use one or two rooms, so as long as those rooms warm, does not matter how long it takes to heat rest of house, it can take 3 hours and would not matter.
So in real terms if you can select rooms to be heating when you get home then no need to control central heating while away from home, a 24 kW central heating boiler does not take long to heat one room.
