I have fitted two MiHome eTRV's and not that impressed, it is not the eTRV at fault, it is down to house design. Nest will work with the MiHome eTRV with a follow command so works similar but not as good as the EvoHome. Well it's hard to say if the MiHome is not as good as new apps are being released all the time, it is an ITTT device.
The basic idea is a eTRV gradually alters the water flow so it is not digital i.e. on/off, but analogue slowly opening or closing the valve, with a condensate type boiler the flame height varies according to return water temperature so during the heart of Winter it will auto adjust output to suit the eTRV demand, the anti-cycle software of the boiler should in theory mean even as the weather warms up, it should still control the rooms at the set temperature.
However there are two problems, one is as summer arrives the boiler would still need to fire up every so often to find if there is a demand for heat, and two when the valve alters the set temperature if the boiler is off then nothing happens.
Taking number two first, I found the time switch can be used to over come this problem, I wanted the temperature to raise at 7 am so I set time switch to switch off boiler at 6 am and back on at 7 am this means the boiler is nearly always running when the temperature set point changes, that worked well.
To stop it firing up in summer you need a wall thermostat, set at a temperature above that of the eTRV, theory OK, room thermostat set to 20°C and eTRV set at 19°C and only on a warm day will the thermostat activate. However it would seem with the follow command with Nest the eTRV and Nest are set to same temperature, so it needs some very careful positioning of Nest to ensure the eTRV works before Nest.
At the moment I am a TRV short, so as yet not fitted Nest, just the two eTRV's down stairs, and the hall has a standard wall mounted thermostat, in theory the two rooms should be independent of hall temperature unless doors left open, so mum is in rest bite so her room set at 12°C and door shut, current temperature 18°C, garage temperature 13.2°C and outside 14°C and the room has been set to 12°C for three days. So in theory it should be no warmer than 14°C, so why is it at 18°C?
Radiator should be stone cold, however it feels neither hot or cold it is hand temperature, but the rails of the stair lift are cold to touch, so clearly even set to 12°C it is still getting some heat, every radiator in the house feels the same, just look warm, now the instructions do say about fitting a coin between the pin and activator if they don't turn off, so maybe it needs that fitting? Having fitted the coin the valve has opened and closed so don't really know if working correct as the radiator has warmed up due to re-fitting the valve. Current room temperature is shown as 21°C.
If and that is a big if, the reason the radiator was tepid is because it needed the extra spacer (1p) fitting then may be now it will work? However I had followed the instructions so it should not have needed a coin spacer. And they have not worked as expected, OK reason was the house has a bay window, and the morning sun hits that bay window and the living room temperature can easy hit 26°C even in the winter, but cloud cover and the room can get rather cool, the other room also has a problem due to wind direction, the room still has a chimney and compared with hall where the wall thermostat is it can get rather cold. The better control of the eTRV has improved the general temperature control of the room. The eTRV were fitted because I had control problems, not simply a whim, so it is understandable that the room is still not perfect.
Until today I blamed the room getting too warm on the heat retained in the fabric of the building. And to be frank that must be part to blame, as with the central heating off completely, (due to an error) in heart of winter, it was some 6 hours later that I realised the room was a little on the cold side, and also it does not matter if the target temperature is 19°C and actual is 17°C if the main thermostat has switched off the boiler it will never warm up.
The hall temperature is a real problem, the kitchen and wet room lead onto the hall, both have independent heating, so hall can get too hot for boiler to run. In my own house, up to now talking about mothers house, in my own house it was designed to be heated with a single central gas fire, it is an open plan and up stairs can get really hot, but the Myson radiator circulates the air, so as long as bedroom doors are kept closed it works well from a single central thermostat. Bedroom temperatures are limited with standard TRV's so it has worked A1 for years. No need or point in fitting eTRV's it simply does not need them. Also the boiler is old, so no option boiler will switch on and off, it does not vary flame height.
So really I need to look again tomorrow, and see if mothers bed room is cold at 12°C if so then the other room also needs a 1p spacer fitting. It must have been nearly fully closing the valve as radiator only tepid, but until mother was not in the house I had no need to set so low, so thought tepid was correct, only today when room unused for such a long time did I realise there was a small problem.