Drayton Wiser with heat interface unit

That does seem encouraging. looking at the pictures you have the standard Brown (L), Blue (N) and Yellow/Green (E) wires going into the terminal block which would provide your permanent 240V supply to power the Wiser Hub. Presumably the brown & black cores in your initial photos from Tuesday 20th are distinct from those in todays photos.

At first sight it should be a case of running a 3-core cable from the HIU terminal block to the Wiser back plate to connect to L, N & E. Then connect up the old thermostat brown & black to Wiser 1 & 3. You may wish to install a switch between the HIU & Wiser to allow you to power off separately unless the HIU circuit breaker is suitable.
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Thank you - that makes sense to me. One thing I find confusing still is that the wiring diagram mentions that a 3A fuse is required. I initially thought that this would be on the L but looking more closely at the diagram and the instructions, they say that “power should be put on to Terminal 1 […] from a separate supply with a 3A fuse”. In my case the existing brown and black wires are the “separate supply” and this implies that the fuse should be between the HIU and Wiser, and not between the HIU terminal block and the Wiser. Is my interpretation correct?
On a normal heating system the mains power supply which goes to the boiler, heating controls, pump, valves, etc would originate from a single source protected by a 3A fuse. The diagram is just saying the power source which goes to Wiser L & N needs to be protected by a 3A fuse.

In your case you only have the Wiser Hub. So in theory you could provide a regular socket connected to your HIU. Then put a 3A fuse in the plug that goes into the socket. Or something like this

but replace the fuse with a 3A one.

The dotted line in the diagram between L & 1 is only required when terminal #1 requires 240V. In your case you should not add this link. You should view the mains supply going to L & N (plus Earth) is completely separate from the brown/black cores going to 1 & 3. View it as a solid dividing wall between them, L, N & E on one side and 1 & 3 on the other side.

In essence on a single channel system such as WT714R, the Wiser is joining terminals 1 & 3 together (heating-on) and disconnecting (heating-off). no power is being added just the circuit opening & closing.
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So the Hub’s now wired in and it seems to be working very well indeed, with the thermostat finally being in a sensible place in the living room. Thank you all so much for your patient and detailed help!
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