As
@flameport has said with a system boiler the thermostat/programmer controls motorised valves which in turn control the boiler, there were some 24 volt versions but very rare, in the main 230 volt, but with the modern gas boiler there are three methods of control 230 volt, and an extra low voltage often 24 volt and a variable voltage connected to ebus of boiler. Although the latter does have the option for a common protocol called OpenTherm not all boilers have the option, and Hive does not have the option for connection to ebus.
I personally feel using the same back plate for volt free and 230 volt is flawed, it is two easy to fit wrong thermostat/programmer, but the whole industry seems flawed as to central heating control, there are two methods to split the home into zones, one is to used motorised valves as stand alone units, so you have just two areas, the other is to use thermostatic radiator valves (TRV) and motorise them instead.
Drayton make a 3 channel programmer, but it seems only EPH makes OpenTherm thermostats which can be set as master/slave so both the boiler output is controlled and the stand alone motorised valve is operated. However this means a whole group of rooms controlled together, and the TRV heads can't be linked to the wall thermostat. The only other thermostat/programmer which does not seem to link to TRV heads is Nest (But it does have OpenTherm).
Drayton Wiser, Honeywell EvoHome, Tado all seem to be OpenTherm or have option for OpenTherm add on plus connection to the TRV heads, but some boilers are not OpenTherm enabled and either don't have option for ebus connections or use a special, Worcester Bosch for example.
But the boilers are designed to also use return water temperature to control output, and Hive is designed around that, so it uses linked TRV heads and a "demand for heat" to give individual room control without having OpenTherm or stand alone motorised valves. It was designed so you have a single wall thermostat for the whole house.
There is nothing to stop you having 10 room wall thermostats in parallel one in each room, so the wall thermostat ensures the boiler runs, and the TRV head stops room over heating, however this means the temperatures for wall thermostat and TRV need to be carefully matched, so far easier to have one device for each room, the TRV.
I have a TRV and a wall thermostat in my hall, but they are some distance apart and the wall thermostat is higher, so summer they both read the same temperature, but today at 3:30 am TRV is reading 17°C and wall thermostat reads 19°C the boiler is not running and has not been running for a couple of hours, but nothing circulates the air between the two thermostats, so I have to allow for the difference in the settings on the two schedules, but the difference reduces as summer arrives, so never perfect. My wall thermostat is claimed to be "Smart" and it tells me at the moment it is raining, but it does not know which doors are open or closed so some times it does not fire the boiler when required. This is where Hive is better than my Nest, Hive connects to TRV heads, so all Nest's clever algorithms can detect if we are at home, but not is door left open. Hive may not have a PIR built is to see if anyone is moving around, but it does connect to Hive TRV heads.
But having two Hive wall thermostats controlling the same boiler is rather pointless. The Hive TRV heads tell the wall thermostat when to fire the boiler.
OK I also have two stand alone motorised valves, so there is a case some times to have two wall thermostats, this is when there is more than one pump, in many boilers today the pump is part of the boiler, and variable speed, but my boiler heats both main house and the flat but there is a separate pump for flat and house, so I need a way to run the appropriate pump and this is done using the zone valves. But this is unusual and also my boiler does not modulate it is oil fired.
So it seems where you have a single pump either you just connect both stand alone zone valves to work together and use the TRV heads to control each room, or you fit Drayton or EPH controls, as it seems only those two can control two central heating zones.
In my case flat rarely heated, so seems pointless including it in the main house heating, may be 6 days per year the flat is heated. But of the 7 rooms in main house, 4 are in daily use, two upstairs and two down stairs, and the other three only when some special being done, spare bedroom down stairs and an office and craft room upstairs, so I use motorised TRV heads, so I can select which rooms to heat and when. Not that it really works, as not trained my wife to close doors, I have time yet she is only 67, what I don't have is the nerves to attempt it.
So all big ideas, 9 electronic TRV heads, so 9 independent zones, but doors left open so all pointless. Same with geofencing, we don't go out much, but it only turns heating off if both phones leave the house, so drop wife off and say give me a ring to pick you up, and she says I haven't got my phone. So was there any point having geofencing? We spend less than £2 a day on heating, so last year in winter we left the house for more that 5 hours may be six times so geofencing has saved us around £3 even if I live to 100 it will never pay for its self.
So yes going the Hive would likely be better than Nest since not a modulating boiler, but would never get the £600 back so what is the point?
