Zones are a problem with modern heating as the old zone system was simple on/off but the modern boiler modulates (up/down) with on/off you can have many devices in parallel and if any one switches on the boiler will switch on, but with connection to ebus only one device connects to the boiler, but you can have many devices connected to that device.
Not all boilers give the option to connect to the ebus, so thermostats like Nest and Tado allow one to select old digital (on/off) or modern analogue (up/down) not fitted Tado, but Nest is designed so it will fit nearly every boiler on the market. At the moment Nest has a problem, since google took over Nest the support for MiHome Energenie has been withdrawn, so it does not have a TRV head which can pair with it.
However any good modern system requires each room to be independently controlled, not a block of 4 rooms, but each room on it's own, there are two basic methods, one put the radiator in a box with a fan and control fan speed, two fit a thermostatic radiator valve to a panel radiator, in both cases when the radiator is putting heat into the room, the return water gets cooler, either way it is the radiator which heats the room and be it a programmer/thermostat controlling fan speed or TRV opening/closing the zone valve has had its day, and is now redundant, however still fitted in many new houses because people think it is needed to comply with building regulations, even if when fitting them it completely messes up the boilers built in controls.
So the wall thermostat be it analogue or digital can work in two ways, as a hub collecting information from the TRV heads, or as a anti-cycle device designed to switch off the heating when we get warm weather, it does not actually control house temperature on its own, it can't it is only fitted in one room/area.
There is a method where a TRV head and wall thermostat are carefully matched, this is how my house is controlled, the wall thermostat is in the hall, and there is also a TRV in the hall both set to same schedule, idea is, if door opened the TRV is cooled rapid, so it switches off for set time, allowing one to unload shopping from car, after set time it opens to re-heat the hall, but before it reaches the wall thermostat target temperature it starts to close, delaying the wall thermostat switching off so the boiler heats rest of house. It has been found the TRV is always cooler than the wall thermostat so not the same temperature, TRV set a degree or so lower than wall thermostat, but they change temperatures at the same time.
No two houses are the same, the advantage with systems like EvoHome is they will work without a lot of trimming in nearly any home, where Nest needs some more thought on how set up. I don't have the TRV head set to geofence, but it could, I only have the Nest set to geofence. In last house it was reverse, the wall thermostat did not geofence but the TRV head did. (Geofencing is setting a distance for you phone so once within that distance heating turns on) with Nest there is also a PIR built into the wall thermostat which will turn on heating if people are detected, you select geofencing and/or occupancy detection or turn feature off.
I have some expensive TRV heads (£50 each plus price of hub) and some cheap TRV heads (£15) you can get programmable heads for less than £10, it is not a case of this head is the best, I actually find the cheap head in the bedroom better than expensive wifi model as if I go to bedroom early press the eco/comfort button and it swaps the two target temperatures, and shows them on the LCD display.
EvoHome shows the temperature on the wall thermostat for each room,
With my system I have to go to phone, tablet or PC to see what the heating is doing
In my case also lights and sockets as well as heating. The problem is until you install often people don't know what it will do. This is one of the pages from the cheap bluetooth
it shows the schedule set.
Point is every programmable TRV head forms its own zone, well the eQ-3 shown above can be paired with other TRV heads so where there is more than one radiator in a room the heads are linked.
Best radiator must be the fan assisted, where the fan speed varies, however the building management system to control them is supper expensive, it can both heat and cool from the one unit, either with one matrix or two one for heating and one for cooling, so one side of building can be heated and other side cooled at the same time. Main point with fan assisted is there is very little in the radiators to store heat, so they react fast, only house turn on to warm house 20 minutes. But look at price of the Myson iVector and you will see why not found much in private homes.
The big question is budget, and second is what is already installed, if you have a Bosch boiler then no point looking at OpenTherm as not supported, and if you have zone valves question is if worth ripping them out, or should you try making a silk purse out of a sows ear? Point is you can make it work, but not quite as economical.
We rarely rip out a reasonable system and replace with really good, we rip out a system because it does not do what we want, so what ever we replace it with, it will be better, finding two identical homes one with OpenTherm and one with simple on/off with identical families living in the home will never happen, so we can never say changing to this will safe you this.
So we use near enough engineering.
