Nest when Google took over removed support for Energenie TRV heads and in the US released remote temperature sensors, but this has not happened in the UK.
I am not sure if the Vaillant Ecotec plus 612 can work with OpenTherm there may be an adaptor, but zone valves with ebus it rare, EPH do a system where the thermostats are set as master/slave, but a TRV forms a zone, so there is no point having TRV's and zone valves in most cases, there are exceptions, I use zone valves for flat and main house, as flat rarely used so want to turn off the group of 4 rooms. But most people have upstairs rooms used for some reason other than sleeping, be it kids doing home work, or use as office or craft room, so the whole idea of upper and lower zones is flawed.
How much ebus control helps I am not sure, but the location of the thermostat will likely change if moving from ebus to on/off. In the main ebus control is in the main room, and there is no TRV in that room, but on/off control is in a room one ground floor with no alternative heating, which includes sun through windows, no outside doors, and keep normally cool which means not the main room, its job is not to control the temperature of the room, all it does is turn the heating off when not required.
It is also common to not fit a TRV in same room, but where the room has an outside door for example the hall, then having a TRV and wall thermostat is a good idea, as it can be set for a quick recovery when outside door opened but the TRV starts to close before the set temperature of wall thermostat so rest of home also gets heat.
The whole idea is the TRV sets maximum heat, I struggled with mothers house, no TRV in the hall where the wall thermostat was, OK she was in a wheel chair so front door when opened was open for a long time, but adjust the lock shield to maintain hall temperature and it would not recover after opening door, and set it to recover and rest of house got cold, the adding of the TRV in the hall transformed the system. OK not easiest to set up, but once done worked well.
But her house gave heating engineers a bad name, it was clearly installed by a load of cow boys, that bad had to get them to return it make the system legal again, they had left a power shower in after converting to combi boiler. If it had been a small firm who did the odd central heating job could have understood the errors, but it was a large firm doing nothing but fit central heating systems using grants in the main for old people.
So step one is to ensure everything has been set correct to start with, I found every lock shield valve wide open. Mother could not reach lock shield in wheel chair so not her.
The problem is two devices the lock shield valve and the TRV do basic same thing, they both adjust water flow, so you need to set one before adjusting the other, which without a differential thermostat or a display in degree C is near impossible, I used an electronic TRV so temperature displayed in degrees C both target and current, and if current exceeded target I closed the lock shield a tad. Installers use a differential thermostat and set it to around 20°C.
9 times out of 10 complains about poor temperature control is down to not being set up correct, not a faulty thermostat.
