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I bought a Nest 3rd Generation Thermostat. Live in a 2 family home, where my floor/basement has the boiler room/furnace. I have a Laars Mini Therm Furnace Model JVT. There's a thermostat upstairs as well, where both control / call for heat when needed. For some reason, my Nest is never receiving Rh power (I checked with voltage meter and shows 24 VAC). I've read countless articles on this site and elsewhere and it seems a Common Wire would help alleviate this however I can't find where to run the Common Wire myself. Not a pro, but I've messed with this furnace for hours this whole week and practically know it but no cigar yet. Can anybody help?

I've attached pictures. I think it goes on the A? terminal however there's no voltage detected there (I once saw 12 VAC btu not sure if it was a fluke)

Thanks everyone [![Terminal Board[][1\ Updated Picture

  • Can you provide a wiring diagram for your system? This is going to be messy to say the least, as I suspect the two thermostats have been wired in parallel... Mar 21, 2017 at 3:57
  • We have plenty of C-wire questions. Does this answer help? Mar 21, 2017 at 10:00

1 Answer 1


Currently according to the wire diagram the wire on the R terminal comes straight from the transformer. The C wire should be the other leg from the transformer. So you need to add a wire branched off from one of the Y wires and run it to the NEST thermostat.

  • Note that "Y" on the furnace wiring diagram is not the same as the usual use of Y (which is for cooling). What your furnace diagram labels as "Y" would actually normally be C. The Y terminal on your thermostat calls for cooling, and if you were to connect it to your furance's "Y" (which is really Common) it could cause the transformer to short out.
    – gregmac
    Mar 21, 2017 at 20:00
  • So do I run the Common wire through the screwed in blue screw on the panel? Or twist it into the Yellow electric nut? (I updated the picture above in the main description in accordance to the provided answer by Ratchet Freak) Mar 21, 2017 at 20:33
  • Hey gregmac, so where would I run the common wire through then? On the Secondary side of the Transformer, I see the Red Wire provides the HOT 24 VAC for the rH. But the Y terminal would cause the transformer to short out? (I assume short out is bad) Mar 21, 2017 at 20:38
  • Also, is there anywhere to determine the common wire is effectively continuously outputting 24 Volts with a Voltage Meter? My Voltage Meter shows 24 VAC with Positive and Negative indicated. Would I look for the same thing to confirm the Common wire is giving power properly? Mar 21, 2017 at 20:40
  • Looking closer at the photo and diagram, I would take the common from the yellow wire nut. You can test with your multimeter between that and the Rh terminal (though note with AC there's no positive and negative like there is with DC).
    – gregmac
    Mar 21, 2017 at 20:46

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