There is no provision to power my TRV heads from the same supply as the boiler, nor is there an option for hard wires LAN, it has to be battery powered and has to use wifi or blue tooth. So I have 10 separate supplies to my central heating.
I have in other house a battery powered wall thermostat, so again two separate supplies. Be it 3 volt, 12 volt, 24 volt 48 volt or 230 volt the supplies are not linked.
Running 5 core cable from the boiler to the thermostat with 24 volt on two cores and 230 volt on the other two cores with the latter being AC is not really what should be done as the capacitive and inductive linking in the cable could result in over 50 volt above earth being present in the extra low voltage cables, you would need to run them as separate cables with either steel between or some distance.
OK I know we all do use 5 core, but that does not make it right. I will also agree where possible I would use one FCU to supply all central heating, this house had three FCU on the central heating plus a 13A plug, and I rewired so now they all come from same FCU, where before not even from same consumer unit. But I have worked with many machines with duel or more power supplies, in fact my central heating in all houses had at least two, being supplied with both gas and electric, or oil and electric. Does not matter if air, gas, oil or electric they are all power supplies.
So it was common to fit a valve so if the electric was cut, so was the air or gas, however even this needed so thought, having an emergency stop kill the air supply can result in something dropping, so often twin air supplies, one which is not cut when e-stop is pressed. This is a safety feature decided on after doing a risk assessment.
So in this house, if the supply to the front of house ring is lost, I can't turn off my Christmas lights at the back of the house, as the socket powering the lights is wifi controlled and router is from front of house sockets, so no router, no control. Same applies to lights, or any other wifi controlled system. So even my TV has multi supplies, as to work it needs router, main sky Q box, satellite sky Q and TV all to be powered, and of course a satellite signal.
With a volt free thermostat the 230 volt power is nothing more than a battery eliminator. I do see the problem where some one replaces a volt free with an non volt free, but we can't really stop some one doing this, all we can do is put a warning sticker as some one working on the thermostat may prove dead but a timed supply could then make it live again.
BS7671.2008 said:
537.1.5 Where an installation is supplied from more than one source of energy, one of which requires a means of earthing independent of the means of earthing of other sources and it is necessary to ensure that not more than one means of earthing is applied at any time, a switch may be inserted in the connection between the neutral point and the means of earthing, provided that the switch is a linked switch arranged to disconnect and connect the earthing conductor for the appropriate source, at substantially the same time as the related live conductors.
Note if from same earthed system this does not apply. Maybe changed with BS7671:2018? but it may be bad practice, but can't find a rule to stop using two FCU from the same supply, and not many houses have two independent supplies.