English is a problem when writing instructions with little or no ambiguity, and to ensure the device has volt free contacts and enough isolation is a problem.
However in some cases we know there is a problem running AC at low voltage and DC or AC at extra low voltage close to each other, so in some cases it would be better to get a local supply.
I have to agree this house when we bought it having the central heating supplied from two independent distribution boards was bad, and if following the instructions
@stem has kindly shown then it would not comply as two independent double pole isolators, one on the fuse box and one on the consumer unit, however that is rather rare to have two distribution boards with no common isolator, in the main to lock off with a FCU we put a lock through the fuse carrier, but that only locks off the line, I don't know if the switches on a FCU are always double pole or not, and common not to have a switch on a FCU anyway.
So we know that putting low voltage AC with other supplies within a cable can mean inductive or capacitive linking, and some boilers actually put the low voltage and extra low voltage on opposite sides of the boiler, clearly so one uses two independent cables.
The writers of BS 7671 often change the wording as clearly they feel it is being miss read. There is no where near that care taken when writing boiler instructions, so when we read them, we do need some common sense, so "Any additional components that are connected to the appliance that require 230 volts must be connected to the same supply as the appliance." would be very hard not to comply with, yes I do have inverters, and we do have solar panels etc. But to run some items off and inverter or generator and others from the DNO supply would take a lot of doing, so that would simply not normally happen anyway.
So we are looking at English, it does not say same circuit, it says same supply, had it said "Any additional components that are connected to the appliance that require 230 volts must be connected to the same circuit as the appliance." then that would mean same fuse/MCB/RCBO but same supply means not having part of it running off a generator.
The Ideal saying the isolator should be near to the appliance makes some sense. I have never studied if all switched FCU switch both lives, but to be frank I would prefer a plug and socket, once you have fitted one of these
no one can turn it back on, and your not relying on a switch contacts. With a FCU you can put the lock through the fuse holder, but that is only isolating the line.
The
Type A RCDs
must be employed where additional protection is required.
is interesting, I would assume that means if a TT supply rather than TN supply, but why not say "Type A RCDs
must be employed where the supply is TT?
It does seem central heating uses Chinese whispers as main instructions, I would say any motorised valve makes an area a zone unless linked, I know the eQ-3 bluetooth TRV heads can be linked to make whole room one zone, but a TRV is a motorised valve, be is a motor powered by the expansion of wax or a pair of batteries, it is a motorised valve. But it seems many people only think a large 15 mm piped valve is classed as motorised, however some people do seem to think if no electric powered it not a motor, the number of air motors and activators I have found which still work when the e-stop is pressed I have lost count.
But now we only have to take manufacturers instructions into account, and don't need to follow blindly, why should a supply for a volt free thermostat be taken from the boiler?