I did not see
@AndyPRK post when I posted, I have 9 programmable TRV heads, 4 are Energenie MiHome, and 5 are eQ-3, the eQ-3 are bluetooth not wifi, and will only connect to one phone, but can be controlled from the device its self, cost me £15 each, the Energenie are wifi and need a phone or PC near impossible to control direct, originally they were designed to work with Nest, but Nest withdrew support.
I used the Energenie in the last house with gas central heating and a modulating boiler and they worked spot on, once set, the display shows
the target and current, and if current is over target close the lock shield a tad, and if under open it a tad, and in the old house once set they were spot on, however new owners did not want them, so put the old wax type back, and guess what, now set, they also worked spot on. So yes
@AndyPRK is right in a way, not required if heating all rooms at the same time.
I will admit in this house with oil boiler which does not modulate not quite as good.
But the programmable TRV head not only controls temperature, it also controls when, so if out at work and returning at 6 to first cook meal, then eat it, then sit and watch TV then go to bed, you can set the TRV heads to same sequence, kitchen warmed first, then dinning room, then living room, then bedrooms. This means as long as radiators are big enough you can heat rooms faster as only heating that room, and not heat rooms not in use at that time.
Sounds great, but in practice I don't shut doors, so all this clever technology is wasted.
The video
@AndyPRK links to says what I have also found with Nest, it is too clever for its own good, and changes the scheduled heat patten without you doing anything, claimed to be a learning thermostat, but needs to go back to school, I had to turn off the learning features. The geofencing with Nest does seem to work, but also the TRV heads have geofencing so not really required.
As to how to modulate the boiler, not so sure, whole idea of OpenTherm is the thermostat turns the boiler up/down, and the result in up/down rather than on/off is a smaller hysteresis, i.e. it does not over shoot, we found in old house sun on bay windows could cause the temperature to raise way over settings typically hit 32°C before the radiator had cooled, keeping the radiators cooler, using the programmable TRV and modulating boiler also means they stop heating faster, so still could hit 25°C when sun came out, but that was far better, it was due to bay windows, they were a sun trap, but reaction speed is important.
I has a stupid Bosch boiler so OpenTherm not an option, however by fitting both a wall thermostat and TRV in the hall it could be set to work nearly as good. Idea is open front door and TRV opens reheating the hall, but before it switches off the wall thermostat, it starts to close again, so the wall thermostat only gets warm enough when the weather is warm to turn off, it stops the boiler cycling in warm weather, that's it only job, room temperatures are set by the TRV heads. It does mean the hall TRV and wall thermostat need setting to work with each other, but did it in last house so can say yes it works.
However that means having an expensive wall thermostat rather pointless, if the wall thermostat is programmable also the TRV head needs to be programmable so they match, but don't need to be linked, linked is easier to set up, but not needed.
So it is the TRV head that does all the work, so look at cheap first, the Terrier i30 and the eQ-3 both not wifi, the eQ-3 has two versions one with and one without bluetooth, the bluetooth makes setting easier but is not required.
So this
has two buttons plus a dial which you can also press so I have right hand button set to alternate between eco and comfort setting, so I walk in room and press button and it goes from 17°C to 21°C and at next scheduled change back to 17°C, simple, no phone, not wifi, can use phone but easier to press button. It also has open window detect, so in kitchen when we open door to unload car it auto turns of heating for 15 minutes while we unload. Since leaving EU price has gone up, but I paid £15 for bluetooth and seen non bluetooth at £10 each.
Big question is why do you need a wall thermostat? All it does is switch off heating on a warm day, and I have fingers. TRV set room temperature, so what does the wall thermostat do? A Sonoff wifi switch can switch heating on/off as you approach or leave home.
