2

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

5
  • 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

0

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Browse other questions tagged or ask your own question.