My wifi heads exercise the valve does not matter heating on or off, not a clue with wax, as to TRV flow or return it depends what you want the TRV to do, in my case the TRV changes the temperature three times a day, and keeps the temperature to within a deg C of the settings, to do that they really do need to be on the supply side so they start to react as soon as the hot water arrives, if fitted on return the radiator fills with hot water before it is sensed by the valve, so is more likely to over shoot, however careful adjustment of the lock shield valve can cause a delay in radiator heating which in turn gives time for the valve to adjust, it just takes a little more setting than on the supply side.
The problem is not what the valve can do, but knowing if it's the lock shield valve that needs a tweak, or the TRV that needs a tweak, when for a time I moved out of this house and swapped the wifi valve heads back to wax it worked nearly as well as the lock shield was set correct.
But if a room is too warm, unless you have a report target 20°C current 22°C how do you know which valve to alter? Knowing room at 22°C and valve set half way between 3 and 4 simply does not cut it.
However the wifi is really not required, I know at around 10 pm I will be going to bed, so at 8 pm bedroom temperature goes up to 18°C and at 10 pm living room goes down to 16°C, I get up around 8 am so at 6 am the living room set to 20°C and at 9 am bedroom down to 16°C. This is very rarely altered, there is no real advantage being able to alter temperature with PC or Phone. Likely the Terrier i30 would have done the job just as well.
As for geofencing, because the heads have anti-hysteresis software to stop them over shooting, the last 2°C is slow so set at 16°C and then moved to 20°C will hit 18°C reasonable fast, but a couple of hours before at 20°C which is rather pointless as house rarely cools to 16°C often only dropped to 18°C so unless you work 100 miles away, geofencing is pointless, times are far better.
The ideal is to control the boiler with a modulating hub, if not that then a modulating thermostat, but if the thermostat is not a modulating type, then it's only job is to stop the boiler cycling in the summer, and you don't then want any anti hysteresis software on the thermostat, having 2°C between off and on is good, as whole idea it to stop cycling. This means wifi off/on wall thermostats are out, as either they have no fail safe, or have anti hysteresis software, so it has to be hard wired type.
With an exception, although gas boilers modulate possibly 6kW to 28kW oil often don't modulate, and if they do more like 18kW to 24kW so unless using a heat store, if using oil then the TRV does not work anywhere near as well as with gas.
So with gas whole idea is boiler output varies rather than it switching off/on so TRV is very important, with oil it will switch off/on anyway so TRV only stops rooms over heating, they don't control the boiler either by return water temperature or direct connection to boiler bus.
if not there is something fishy going on 
