As
@muggles says, you have option of using OpenTherm, the new Hive, and Drayton Wiser have OpenTherm but not all Hive models, Tado the same, some have OpenTherm, but not all, I think EvoHome also has OpenTherm.
Assess to terminals should be permitted, as electricians should check when doing an EICR, but some boilers it seems to do put the terminals inside the sealed part of the boiler, and often no signs to show what needs gas safe, and what an electrician can access. As an electrician, I was taught how to use flameproof fittings, but that did not make me gas safe. Could work on a gas expiration ship, (which I did) but not on my own central heating, seemed a bit off to me.
However, the question is what are the best TRV heads. And do you need OpenTherm? I expect
@muggles will tell us. The main change with central heating is to heat each room only when required, and only just warm enough, this clearly can't be done with a single thermostat, with open plan homes, and hot air systems this is not really an option, but in my home, 14 heated areas, I include the hall in that count, and some areas either only used on some days, the craft room and office for example, or only used as set times, bedroom for example, so the programmable TRV allows us to select when a room is heated.
There are a number of types, stand alone eQ-3 and Terrier i30 for example, the eQ-3 also do a bluetooth version so two radiators in the same room can be paired, then we move to internet connected versions, I use Energenie and Kasa, these also need a hub, and then the linked to boiler version, I have one Wiser TRV which does that.
Theory sounds good, but I found radiators next to outside walls get colder than the room in general, so a linked TRV head can fire the boiler when not required, once the thermals are established it does not matter, but some rooms wall thermostats work better.
It is unlikely you need a wall thermostat or linked TRV head in every room, but also unlikely the home will work efficiently with just one thermostat, clearly if the living room is turned off overnight, not much point having a single thermostat there. But one in the coldest bedroom will ensure boiler runs when required, so other bedrooms don't need the TRV linked.
So in my main part of the home (only 10 heated areas) I have 3 thermostats which can fire the boiler. So in the main, we look for a flexible system, where we can easily add extra wall thermostats or fit electronic TRV heads if we find the need. I am not saying go out and buy an all TRV head controlled system from the word go, you can buy kits
and with the Wiser there is in theroy no need for any wall thermostats. But if you buy one which can be expanded, then you have the option latter if you find you need it. From memory Wiser will allow you to have up to 9 wall thermostats.
Nest for the USA market do sensors so you can monitor multi rooms, but not for the British version, will not even link to TRV heads, so a non starter. Kasa the hub and thermostat are one unit, which needs hard wiring, Energenie was the same, so neither would work for me. Hive has changed, and I have not kept up with what it can now do, early versions did not have OpenTherm and would not take a demand for heat once the wall thermostat was over 22°C, EvoHome was one of the first and you don't need a smart phone to make it work,
it has a central control panel for all rooms. The Wiser TRV heads are claimed to work out when they need to turn on to get room warm by a set time, I have as yet not done a winter with Wiser, so can't say how well they work, but to date seems good. Nest Gen 3 was in the main a flop, it needs a power supply to the thermostat, be it from the heat link, or USB and to get an UPS supply to the USB seemed too much like hard work, so used existing wiring to power from the heat link, which in turn like the boiler has battery back-up but it fixed the location to the hall. And the hall cools too slowly. I still use it to get DHW if required, but the Wiser wall thermostat is battery powered, so will work in a power cut.
I do have a problem with the Wiser TRV head, as out of range to the hub, so needs a smart socket adaptor to relay the info, so with a power cut it will fail, however two other thermostats will continue to work, so will not be left in the cold, and I do have a portable battery pack which if required could power the socket adaptor as simple plug in.
My boiler (oil) is not OpenTherm, and I don't know how much it helps. In fact, my boiler does not gain the latent heat from flue gases, so I will leave it to those who know, to say what it can save. The main point is for a central heating system to work, does not mean it is working efficiently, and unless one has had both systems in the same home, one will have no idea. I expected when moving to this much larger house to my last one with oil rather than gas for my heating bill to go up, in fact it has gone down. I think likely due to old house being open plan, and this house has internal doors, but only a guess.
