Nest Heat Link

Wiring diagrams:

From eg p20, it looks like it's basically a relay contact with call for heat

- ie live in wire from boiler, switched live out to boiler, earth.

If that's the case you could potentially replace it with a basic wall thermostat, which would give you temperature control and the ability to turn it off completely. If you wanted time controls you'd need a programmer plus a thermostat in series.

I'd try putting a switch between live and switched live pins and see if the boiler fires the heating. If so you can replace it with a thermostat.

Theo

If you need a timer and a thermostat, better to get a programmable thermostat (about the same total price) which will let you set different temperatures at different times of day/week.

Thanks, that explains a lot, and the diagram shows that my green/yellow wire isn't earth after all, it's call-for-heat. The use of self-adhesive trunking, unstuck and poorly cut, shows that this was installed by someone less than competent. It does make me wonder about the rest of the house wiring, I've already replaced bits of unstuck plastic trunking elsewhere.

It's highly dubious (though for some reason not outright forbidden) to re-use green/yellow wire for a switched live like that, at least it should have been overmarked with brown (or red).

That surprises me, I would have thought it was strictly forbidden. I'll replace it anyway.

In my experience with self adhesive trunking never sticks for a long period I always add a few screws/washers.

This is the type of cable you need. It has four cores L & N feed with another core for switched L and Earth.

Screwfix £17 for 5m (£31 for a 25m roll)

For a boiler/pump etc. you may get away with 0.75mm2 wire (6A, 1300W) 4 core. £8.40 for 10m @ screwfix)

TLC approx £1.30/m (incl VAT) for 1mm2 4 core

In the end when wiring up my CH circuits I had plenty of T and E so used a couple of those to get 4 wires plus earth. When I ran out of te cabpe that connected the intermediate lighting circuits together.