I got a nest thermostat which requires to get a C wire. My old thermostat does not have c-wire connected, but there are four unconnected wires as shown in the picture. unused wires behind my thermostat

Then I came to my furnace and found there are two unused wires (orange and blue) near the control board as shown in this picture. The multimeter measures 26 volt between R and Com. First I put the blue wire under Com and measured only 15 volt at the thermostat side between Rc and the blue wire. Then I disconnected the blue wire from the comm terminal and put the orange wire on. I measured 25 volt at the thermostat side between Rc and the orange wire. Why do the two wires give different readings? enter image description here

Then I realized my HVAC is a zoned system. It has a zone panel. This is the zoning panel diagram. enter image description here I also found unconnected orange and blue wires in the zone panel. Using my multimeter, I found the blue/orange wire in the zone panel is connected with blue wire/orange in the HVAC control board. At the bottom of the picture, you can see the wires connected to the thermostat based on my understanding. I don't see any orange/blue wires there. enter image description here

So I got totally confused.

  1. where is the blue/orange wires behind my thermostat connected to in the HVAC/zone pannel?
  2. If I want to have a c-wire, what should I do? Should I connect a wire to the HVAC control board Comm terminal or somewhere on the zone panel board?

Thanks for your suggestion.

1 Answer 1


Connect a spare wire from the thermostat to the M1 terminal for that thermostat's zone

The wiring diagram posted for your zone board specifies that thermostat C wires should be connected to the M1 wires for their corresponding zones, along with the return from the zone damper motor. So, I'd take a spare wire (black maybe? or purple/blue) from the cable from the thermostat and connect it to the corresponding M1 terminal (far right terminal in the zone block) on your zone board, then hook up your Nest, turn everything back on, and enjoy!

  • Thanks for your help! so does M1 can connect two wires?
    – Yuliang
    Nov 9, 2020 at 5:28
  • @Yuliang -- yeah, connecting two wires to M1 should be fine Nov 9, 2020 at 12:41
  • Thank you for your help!!
    – Yuliang
    Nov 16, 2020 at 18:57
  • 1
    @Yuliang -- we thank folks around here by upvoting and/or accepting their answers :) Nov 17, 2020 at 1:08
  • 1
    @Yuliang please give an up vote and click the check mark. That way others will know that there's an answer here that could solve their problem.
    – FreeMan
    Dec 9, 2020 at 15:57

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