Using same supply for all central heating where does it say we must?

27 Jan 2008
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Llanfair Caereinion, Nr Welshpool
United Kingdom
All parts of a wired control system should have a common power source and isolation point. Sometimes this requires additional poles on an isolator or relays that are clearly labeled and shielded to prevent incorrect connections, ie Fire alarm interposing relays. There may also be warning notices of other voltages sources. All these are safety requirements, but also assist in fault finding, and I can't see any reason why wireless controls/interfaces should no be treated in the same way.

It could make control fault finding very difficult if there was an unknown wireless control switching something on and off.
My house has two of these, one before the consumer unit, and one in the consumer unit. I do have common power source it all comes from the DNO meter and fuse.

And yes I agree labelling is important as is a as built wiring diagram, I drew mine C_Plan_My_HouseD-relay.jpg showing all the wiring centres etc, but there was nothing I found left when mothers house had a boiler change, or when house re-wired, yes a schedule of test results which said which MCB/RCBO feed what, but in spite of agreeing all sockets to come from a ring final so they could be added to later, that was not the case, I found two unfused spurs and one dead socket.

However the fact that tradesmen don't label or make as built plans does not mean the central heating should not use a different supply for boiler, heat link, router, hub, thermostat and mobile phone all which in some way control the heating. It is very common for the thermostat to have a different supply to heat link. Yes you can take a 12 volt supply from heat link to thermostat with Nest, however it can also be completely independent using a USB power supply or 12 volt power supply both which are in turn supplied from some handy socket, which is unlikely to be supplied from the same RCBO/MCB as the boiler.

The power supply being built into a thermostat or independent as in some sort of wallmart is really not a good reason why one needs supplying from same FCU as boiler and the other not. And the manufacturers instructions don't say from same FCU they say from same supply not same circuit but same supply, and in real terms it would be rather hard for them not to be from same supply, most domestic premises do not have generators so they could be from a separate supply.

I can see if a boiler is supplied from an inverter and the thermostat is supplied from DNO then yes the boiler may not switch off, however that is exactly what happened when I used a wireless thermostat, it lost wireless link, and boiler continued to heat the house.

However the problem was the TRV was not set correctly, had it been set correctly the rooms could not have over heated.
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