I'm installing a nest thermostat in my house - it's the UK version with a heatlink. I have successfully wired the heat link into my combi-boiler, disconnecting two existing wires in the process.

I have a thermostat in a different room (Honeywell CM707) that I've removed from the wall. There were two wires connected to it.

This is the wiring diagram for the thermostat: enter image description here

From this I am expecting that the two wires I disconnected from the boiler are the two that connected to the old thermostat.

However: I tried measuring the voltage across the two wires that were coming out of the wall - when the multimeter was in DC mode it registered zero, but in AC mode it registered 12V. Am I right in thinking this means they cannot be the wires that I disconnected from the boiler? IE if A and B in the diagram was where I was measuring the voltage, and T1 and T2 were completely disconnected from anything, I should not get a voltage across A and B, right?

To me this is a given but I was wondering if there was something I didn't understand about the way multimeters work when they were on AC settings!

EDIT: this is the wiring diagram from my boiler - the wires I removed were in the points labelled 1 & 2 (there was no link cable). enter image description here

  • 1
    It's odd on two points--typically this type of equipment is on 24VAC.
    – isherwood
    May 14, 2019 at 13:04
  • Starting to wonder more and more if it's an artefact of the cheap multimeter I'm using... I've updated my question with the wiring diagram from the boiler
    – Ed_
    May 14, 2019 at 13:50
  • You can test your meter on a standard outlet or doorbell circuit. Should read 120v +/- and 24v +/-, respectively.
    – isherwood
    May 14, 2019 at 13:51
  • 1
    Remotely possible you have "induced voltage" which would vanish if you put a load like a 12V lightbulb across the wires. That would be from the parallel wires acting like a crude transformer, getting voltage measurable on a sensitive meter from the AC on the other wires.
    – Ecnerwal
    May 14, 2019 at 13:51
  • @Ecnerwal - thanks, I think you might be right about this. The wires are quite long which I think makes this more likely. I did connect a 12v nest thermostat to it at one point and had no voltage measured at the terminals, but at the time I assumed this is because I'd blown it up! (I hadn't!)
    – Ed_
    May 14, 2019 at 14:52


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