I think there's "shades of grey" involved.
I can't find the regulation I was thinking of, and it's possibly not specific to heating systems. But at one extreme you could have a mains powered thermostat (or control system) providing a control signal to a boiler - while the boiler is off a different supply. In that case, you could knock off the isolation next to the boiler - and get a zap from a supply from elsewhere.
At the other extreme, where there's a radio link, then no risk of that.
In between we have the sort of situation being discussed here - using the volt-free relays in a UFH wiring center, or the volt-free switch in a zone valve to switch something on a different supply. I would say that's OK given two things : 1) everywhere with multiple supplies is properly labelled (537.1.2), and 2) anyone working on the system is capable of understanding what the label means.
Now, it's something I've done in a commercial environment ("many" heat/cool fan-coil units, relay controlled with local supply and centralised timer, relay isolated demand signal to boilers and chiller plants) - it would have been completely impractical to do otherwise. In a domestic environment I'd be "cautious" - lets face it, some elements of some trades have a reputation for struggling to cope with S-plan wiring, do you really think they could cope with multiple supplies safely ?
I can't find the regulation I was thinking of, and it's possibly not specific to heating systems. But at one extreme you could have a mains powered thermostat (or control system) providing a control signal to a boiler - while the boiler is off a different supply. In that case, you could knock off the isolation next to the boiler - and get a zap from a supply from elsewhere.
At the other extreme, where there's a radio link, then no risk of that.
In between we have the sort of situation being discussed here - using the volt-free relays in a UFH wiring center, or the volt-free switch in a zone valve to switch something on a different supply. I would say that's OK given two things : 1) everywhere with multiple supplies is properly labelled (537.1.2), and 2) anyone working on the system is capable of understanding what the label means.
Now, it's something I've done in a commercial environment ("many" heat/cool fan-coil units, relay controlled with local supply and centralised timer, relay isolated demand signal to boilers and chiller plants) - it would have been completely impractical to do otherwise. In a domestic environment I'd be "cautious" - lets face it, some elements of some trades have a reputation for struggling to cope with S-plan wiring, do you really think they could cope with multiple supplies safely ?
