I note the cheaper TRV £10 or £15 with bluetooth eqiva has an offset temperature to allow for the heat from the radiator, +/- 3.5 degrees C max, mainly I think for when multiple valves fitted in one room.
With electronic heads down to
£10 one wonders why people still use the old wax type, I suppose don't want to change batteries?
However it means paying £40 for a head, £30 is just so you can adjust it remotely, so is it really worth £30 to adjust from your phone when away from home?
OK there are other advantages, the Hive TRV states "If you also have a Hive thermostat you can enable Heat On Demand to automatically boost your heating when a radiator needs heat." it also says "Note:
Hive Hub 1 does not support Hive Radiator Valves." but be it Tado, EvoHome, Hive or Nest much depends on how far you going with the installation, boiler type, and local wiring which will work within a reasonable price.
But how does one compare the options, take Hive and Nest as an example, Hive wall thermostat is cheaper than Nest, but the TRV's are much more expensive, and Nest will work with OpenTherm, Hive will not, but Nest the TRV simply follows the wall thermostat where Hive TRV switches on the boiler through the thermostat. Nest both Gen 3 and e are volt free, Hive only single channel is volt free, so both have plus and minus features which take a lot of reading to find out.
I like the ability to connect Nest thermostat to heat link hard wired so not relying on wireless link, but it often depends what wiring is already installed.
But back to question, how do you test? Or even should it be tested? Well the Nest heat link instructions say the earth is only required when the thermostat is hard wired, so I read that as earth is required when hard wired thermostat is used.
At the heat link easy to test the earth, but at the wall thermostat, first problem is how do you gain access, I know the round lump simply pulls off, but unless you have fitted one, you would be a brave man just to tug. Then you need to find another earth to test it to.
So the box of adaptors gets larger and larger, where do you stop? This house I had to dig out the living room ceiling rose to change it, OK to change it, but just to test? It now has a Maestro plug in ceiling rose, so all you need to do is unplug the chandelier, and plug in your test adaptor, if you have one, otherwise you have to remove plug from chandelier, now a Hager Klik ceiling rose is only £4 so a spare plug for them OK, but the Maestro is £23 having one of them in tool kit is unlikely.
And how many instruction manuals are you going to read? As said Nest is class II unless thermostat hard wired, so what about the rest, just because there is an earth terminal does not mean it has to be earthed.
When I did my C&G 2391 all those years ago, it took me around an hour to test the board used for the test, at that rate it would take a day even for the simplest of house. To just test one ceiling rose earth one needs a set of steps, and if using an electronic switch getting power not easy.
If I said to test a house it will take 12 hours at £15 an hour I don't think I would get much work?
So big question, is the boiler, thermostat, programmer etc part of the installation, or is it an appliance? Does it come within the remit of doing an electrical installation condition report? Supply to boiler yes, everything supplied from the boiler, not so sure.