Define "Smart" it seems some people think "Smart" means it connects to a mobile phone, others is has some algorithms (what ever they are) which record information and use it latter (learn).
So if the TRV is to control room temperature the boiler needs to run, this becomes more important if the TRV is programmable, but there are many ways to do this.
1) Wall thermostat drops 0.5°C before the change and back up 0.5°C on the change.
2) Use a modulating boiler so 90% of the time it is running.
3) Wifi link to some wall thermostat/hub.
4) Some other method.
In theory the modulating boiler does not need a wall thermostat to control it, the return water temperature can do most of the control, with one small problem, once the water stops circulating there is nothing to tell the boiler to turn back on, so best option is some form of anti cycle software, having written PLC programs it would be easy enough to write a program which extends the off time if return water get hot in under a set time, and decrease off time if not, but this relies on the installer fitting a TRV to every radiator and the by-pass valve being close to the boiler.
There is no way to insure the installation is as expected, a standard TRV has around 3°C between fully closed and fully open, so the room would be 3°C cooler in heart of winter to when central heating first turned on, and fan assisted radiators control output by fan speed not water flow, and boiler manufacturers want their boiler to work with all.
So we need some form of wall thermostat to turn off the boiler when not required, and turn it back on when required, this is not to set an area temperature, it is to decide when the boiler is required to run.
As said a mechanical TRV has around 3°C between fully on and fully off, so in theory a wall thermostat set some where between that 3°C window can be used to control the boiler, in practice over shoot can mess this up, so the lock shield needs careful setting, and having a few wall thermostats in parallel can improve how it works.
Nest in USA have these
temperature sensors to ensure heating runs if any room is under temperature, I think, as the way they work seems a little uncertain, but here in UK where most of the heating uses water, not hot air, we tend to have combined the temperature sensor and TRV head together.
To my mind Nest and Energenie MiHome got it the wrong way around, with the wall thermostat telling the TRV what to do, most the TRV tells the wall thermostat what to do, and how also varies. Hive for example has a ceiling of 22°C under that the TRV can send a "demand for heat" and turn boiler on even when the wall thermostat is satisfied. Others use different methods, having a display showing 6 rooms seems good
not a clue how well it works.
But also a couple of wall thermostats in parallel can ensure boiler runs when required and cheap (I paid £15 each eQ-3) programmable TRV heads
can work very well, although will only connect to one phone and only when local as uses bluetooth not wifi. The blue tooth does mean where more than one radiator in a room they can be linked. But other than that likely the manual local controls are good enough, there is also the Terrier i30.
The idea of geofencing seems good, however I found speed is a problem, I can set the eQ-3 TRV heads to come on in sequence heating most important rooms first, and also with the Energenie cheat setting heat to 22°C for an hour then back down to 20°C, but the geofencing I have has no user controls to set at what point it turns heating up. Not a clue if when within 25 miles, 1 mile, or 500 yards, until google took over Nest it did work, but now has failed.
Although when working in Algeria or the Falklands clearly enough time for it to warm my house in North Wales, but the Energenie MiHome TRV heads unless I cheated took 3 hours to reach target temperature, few people work far enough away from home for geofencing to work correctly, specially when no way to adjust the settings.
Picking up the phone and turning it on is really not that "Smart" I do just that with a stand alone AC on a "Smart" socket, I look at the temperature reported by the TRV and if too high turn on the AC when I have finished shopping, mainly because the AC is under size for the room, and it takes so long to cool it.
But central heating control is now all by time, I do have 9 programmable TRV heads, but also a wife who does not seem to understand the concept of closing internal doors, however when she does yes rooms do heat to temperatures set.
But back to the question what is "Smart" does it need to work out the time taken to heat a room and adjust settings to take that into account as it is claimed the Drayton Wiser TRV head does? Or is a simple timed change in set temperature enough to call it smart?
