In an earlier contribution to this discussion,
Mike wrote:

I need to replace my old thermostat, which is cracked, and have bought
a new one. The problem is that it isn't clear where the power source
is supplied from, so I am having problems safely isolating it. Any
tips, please?

I presume you're talking about a room thermostat which controls the central
heating?

If so, the whole heating system - boiler, pump, zone valves (if any) -
should all be fed from a single FCU. If you find that and switch it off, the
stat should be safe to work on - but check it with a mains voltmeter (*not*
a neon screwdriver!) anyway.
--
Cheers,
Set Square
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