What do you call "Smart" TRV heads? My energenie mihome heads can be set to geofence, so as you approach home they can turn on first occupancy rooms, however they only have one button on them and the manual control is rather poor, as far as I am aware they don't have open window detect, however they do have two sensors one for air and one for water and auto compensate for heat from radiator, so when the current temperature is displayed as 20°C the thermometer in room also shows 20°C so reasonably accurate, however the anti-hysteresis software is a bit too good, so move setting from 17°C to 20°C and get to 19°C reasonably fast, but last 1°C rather slow.
Now my eQ-3 heads are very different, and much cheaper, I don't think they have twin sensors, they are not wifi so no geofencing, the bluetooth can only connect to one phone, as with the more expensive you can set a schedule so temperature altered through the day, it does not show current temperature only shows the target temperature, however the head has a easy read display to show this and two buttons and a dial/button so you can simply walk into the room and either select boost, or swap eco to comfort, or dial in a temperature, plus they have open window detect, so are in many ways better than the expensive models.
Clearly the TRV can't heat up room if the boiler is not running, so most important room in my house is the hall, in the hall is both a wall thermostat and an electronic TRV which were claimed when bought to be able to pair together, in fact the pairing does not work, but it does mean the schedule for hall temperature is matched wall thermostat and TRV and in the main the boiler will fire once an hour even if only for a few minutes, and it only takes a few minutes for a radiator which was cold to become fully heated.
The speed of radiator heating can also be a problem, even the electronic TRV takes a minute or so to close, so if wide open, it needs time to close, so the lock shield valve setting is important, two slow and the radiator will not hit full heat in short time boiler runs, too fast and the TRV can't close fast enough and the room temperature over shoots. Also TRV heads work far better on supply side of radiator as they detect the heat faster, they will work on return but the lock shield valve setting is more critical.
With a modulating boiler the electronic TRV head works far better than with a mark/space controlled oil boiler as the modulating boiler turns down temperature and there for runs for longer.
However as said what do you call "Smart" the Hive TRV sends a signal to the Hive wall thermostat with a demand for heat, so even if the room where the wall thermostat is has reached the target temperature the thermostat will still turn on the boiler, EvoHome does not have a thermostat in the old sense, but has a display I think showing 6 rooms, also with the added module it can talk to boiler direct to eBus using OpenTherm system, I think Tado is the same.
The electronic TRV can always cool a room, warming a room needs to boiler to run, so I have two types of electronic TRV, idea is the cheap ones (eQ-3 at £15 for bluetooth version) control bedrooms, office and craft room upstairs plus kitchen, stopping them over heating so when not required those rooms don't heat up, used in kitchen as open window function turns it off when door open and unloading car. The other 4 more expensive (Energenie at £45) were to link to Nest thermostat, in fact they don't link, Nest withdrew support, but they do work with geofencing so can be set to auto turn up at the same time as Nest wall thermostat, and it does work, they did work better in last house with modulating gas boiler than in this house, but they still work in this house.
Hind sight, and hind sight is easy, think the Hive would have worked better, but when I got Energenie Hive did not do electronic TRV heads, so it was not an option. I find the TRV anti hysteresis OTT, so the geofencing is not that good, so tend to set times and a schedule instead, so if not using geofencing then the cheap eQ-3 or Terrier i30 would do the same job, and not having to use phone to switch between eco and comfort I find an advantage.
The bluetooth function does make it easier to set up, but once set, rarely use phone, and non bluetooth version seen for under £10. The expensive energenie does not tell you if the range is too wide or too narrow, but the cheap eQ-3 does, but main advantage is setting in °C not *123456 which was about as much good as a chocolate fire guard.
I think I should have gone whole hog and bought EvoHome, however if I had then I would have left it behind when I moved house, I did look at Hive, but the way the unit is wired you have no assess to com on the relays, they are internally linked to line, and with Nest just two wires between heat link and thermostat takes all info and charges back up battery in Nest wall thermostat, which did suit me.
I have tried to get others to write about TRV heads and what they do, and how well they work, there is very little in the adverts to say what each make does, I got 5 x eQ-3 heads at £15 each, but not seen them that cheap since, the non blue tooth seen at under £10. As to energenie seen a pair for £50 so not so bad at £25 each, but they need a hub, so although good price for me when I have a hub, gets a tad expensive when you need another £40 or so for the hub, to be fair same hub works lights and sockets as well.
In theory I should be able to say "hay google turn up living room temperature to 25°C" and the Nest mini will tell hub what to do, in practice I get reply, don't know how to do that yet. However if I ask "hay google what is the room temperature" it will list out all target and current temperatures of all the wifi devices.
The geofencing and occupancy detect in this house does not work to well, the radiator in hall is too far from Nest wall thermostat and on returning home there is a tendency for it to over shoot. For a schedule temperature rise, I do last few degrees 0.5°C per hour to stop over shoot, can program that into geofencing and occupancy detect.
I only have two electronic TRV heads, would be nice if others using them also did a report on how well they work, I will say I would fit them again, in fact I did I replaced the electronic with old liquid heads in old house and brought them all here. Old house had bay windows, with liquid TRV head, morning sun plus radiator in winter found room at 32°C, sun still could over heat room with electronic heads, but set at 20°C it only hit 24°C which was to me a huge improvement. With a modulating gas boiler in rooms without bay windows and sun problem, the electronic TRV heads kept rooms within a degree of target temperature which was far better than the wall thermostat, or table mounted free standing thermostat could do, thinking about it the radiator causes thermals in the room so at side of radiator is likely the best place for any thermostat, and with energenie having two sensors once lock shield set they were spot on.