Electrical question – supplying power to a single item from two sources?

IANAE so I’m not going to be doing this myself but would get it done properly if it’s possible:

I have electric underfloor heating in the kitchen and it’s power is supplied via a programable thermostat – all very ordinary.

I also have PV and a diverter that sends spare power to the immersion thus cutting down on oil usage – again, all fairly ordinary.

I’m thinking of changing diverter to one that supports multiple ‘destinations’ for spare generation so that, for example, when the hot water was up to temperature any spare power would then be sent to another cylinder, heater or electric underfloor.

My question is, can I use my underfloor as a destination even though it already has a power supply from the thermostat?

(i.e. can another supply be connected to the cable running from the thermostat to the UF mat – is there a possibility of a back feed when the thermostat is supplying power and the diverter isn’t?)

For your PV and Diverter, is it standalone or does it synchronise to the mains supply incoming Voltage and Frequency?
Id like to think the PV inverter output is synchronised to the mains supply.

As to the underfloor heating, yes you could supply from both supplies, but when one is on and the other off, there will be voltage present at the ‘outputs’ of each device (thermostat output and diverter output) – a backfeed. So one safety issue is if someone comes along and isolates one supply, and the other is operational and they dont know there is a secondary feed, there is a risk of electrocution. So lots of orange ‘isolate elsewhere’ stichers on the sources.

The Diverter, is it able to have an ‘input’ voltage present at the ‘output’ and a backfeed onto the electrickery?
In an ideal world, you would automatically isolate one supply, when the other is operational – some form of changeover arrangement.

The Diverter, is it able to have an ‘input’ voltage present at the ‘output’ and a backfeed onto the electrickery?

I shall look into this with the manufacturer.

As to the underfloor heating, yes you could supply from both supplies

Is that;

Yes it will work, is fully above board and conforms with the regs (with some orange stickers)?

Or is it;

Yes it will work, but breaks every rule in the book and potentially lethal to anyone working on the adjacent circuit in future?

I think I know the answer but I’m not a sparky so will withhold judgement!

I shall look into this with the manufacturer.

hmm, well a quick look shows that the diverter can act as a thermostat and takes a signal from the existing UFH thermostat to supply power to the UFH. This means there is only one supply to the mat – which is very likely safer and ‘proper’!

I’m not sure this would work with my theoretical setup so I may just stay with a simpler system that doesn’t include the UFH quite as much as I’d hoped.

Personally I think this would be a pain in the neck and would cost more than you’d save.

Agree with Flaperon, sounds like it could become a bit more complex. Need some form of changeover for Power and control signals from the mains source and the PV source – where the PV source shall be defined as the ‘master’.
The changeover would have to ensure that the other source is isolated from the supply.
Then theres ensuring that the cabling/earthing arrangement from both sources can handle a fault in the other in the event of maloperation/faults in equipment (divertor)

Yes it will work, but breaks every rule in the book and potentially lethal to anyone working on the adjacent circuit in future?

The changeover would have to ensure that the other source is isolated from the supply.

If you have solar, your whole consumer unit has effectively two sources – the mains infeed from the grid and the solar infeed, normally just via a fuse on the consumer unit.

Our 60A main isolation fuse has a massive sticker on it saying “Warning dual supply, disconnect Solar as well as the fuse!”

I have wondered this in the past for electric car charging in the absence of a proper car power point. I.e. Two 13amp plugs.