So called smart heating devices are not really smart if they do not connect to other individual radiator TRV's so that individual rooms or areas can be controlled, and not just a case of switching the heating to the whole home either on or off.
It seems "Smart" means connects to phone, and has nothing to do with artificial intelligence. As far as TRV go there seems to be some very good electronic heads able to control each room individually, but the problem is telling the boiler when it needs to run.
So looking at a good system like the EvoHome each TRV head tells the central hub (called thermostat with EvoHome) if it requires hot water, and then the hub tells the boiler using the bus how much heat is required with something like OpenTherm so the boilers output is modulated to suit the demand.
It depends on which report you read as to how boiler efficiency relates to output, but what does seem fact is the losses each time a boiler is turned off, when it is turned off any heat in the boiler is wasted as it slowly exits through the flue, so modulating boiler is far better than simply turning it off and on.
However there is no reason why you should not have a number of wall thermostats to ensure the boiler is running when required. And the boiler is modulated by return water temperature and all the thermostats are for is to once all TRV heads are satisfied to turn off the boiler.
The big question seems to be, if the TRV head does not electrically connect to the wall thermostats, how should the wall thermostats be set? In my mothers house I have used two wall thermostats, wired in parallel, one could of course fit more, but the thermostats were fitted in an early attempt to control the central heating, before the electronic TRV heads were fitted, I did not disconnect the programmable thermostat, although I had intended to simply use the thermostat fitted in hall, and separate programmer. But found some nights with whole central heating off, it got too cold, so the programmable one was fitted in the kitchen, idea was only used at night, so oven not used at night so having it on side of built in oven would not matter.
The hall was also not ideal as when front door is opened I want a rapid recovery, however that rapid recovery would mean thermostat will turn off boiler before whole house warm, so also fitted a TRV in the hall, set to allow rapid recovery but turn down the radiator supply before the wall thermostat turns off boiler, this does a reasonable job.
The two main down stairs rooms use electronic heads on the TRV's and upstairs simple standard TRV's are fitted. It works, Heath Robinson maybe but it works.
However all down to house, both my own houses both worked very well with a single thermostat, my first house had hot air central heating, and the air is blown into each room and returns through grills in the doors, there was some individually control by moving the louvres on the supply grill in each room, but since air was circulated around the house, whole house was reasonably at the set temperature, very expensive to run, as at that time single glazed windows, so blowing warm air around the room a lot more heat was lost through windows, but it did work with single thermostat. The second house also was open plan, used TRV's up stairs to stop bedrooms over heating, but single thermostat worked well down stairs. Both cases it was house design, not the type of central heating, second house also had fan assisted radiators so air is moved around, but with double glazing the losses are much reduced.
Only in mothers house with traditional separate rooms and doors without vents to every room did I have a problem, and even then the problem was more down to sun shining through bay windows than the central heating, I could get it to work A1 with days where there was cloud cover, but when the sun came out I needed a much more responsive TRV head, and so the electronic heads were fitted.
It is true that fitting EvoHome would have cured problems in mothers house, however not with the other two houses, it is not suitable at all for hot air, and my second house did not have a modulating boiler and the fan assisted radiator does not alter water flow to control output, it controls fan. So there is no one system suits all.
