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I bought for my new apartment thermostat Honeywell RTH9580WF to replace my old Honeywell TH3110D1008. I watched almost every video I could find on internet "How to do it", but I couldn't fined anything to fix my problem.

I have gas heating system/cooling system controlled by one thermostat. My old thermostat doesn't require "C" wire but the new one does. My heating and cooling system are controlled by ICM280 Furnace Control Board witch doesn't have "C" terminal - the one that I need for my new thermostat.

I'm attaching some images where you can see that my old thermostat works with 4 wires - white for "W", blue for "Y", green for "G" and red for "R", also I have jump wire between terminal "R" and "RC" on my old thermostat. On my control board I have only 3 terminals: W, G, R.

My question is where should I get wire for my terminal "C" on my new thermostat? Thanks for any help. Also I'm attaching picture of the same control board as mine but different brand.

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  • Does your house have central air conditioning controlled from the old thermostat? Also do I see the red wire landing on B? The insulation appears to be going to R, but that doesn't count. Also how many wires are in your thermostat cable? Is there a yellow one?
    – Harper - Reinstate Monica
    Commented Feb 8, 2018 at 7:55
  • In the time when I took the picture the red wire was disconnected from the thermostat board and that's why it looks like it's connected to "B". I have only 4 wires come out from the wall: white, red, blue, and green. And also my old thermostat controls the central air conditioner and the central heater.
    – Nikolay Georgiev
    Commented Feb 9, 2018 at 1:41
  • Inspect that cable closely for whether it has a 5th wire. If it does not, then searching for C on the furnace wiring is moot. Unless you could pull another cable easily enough. What is the difficulty scale of pulling another thermostat cable? (there are other ways to do it.)
    – Harper - Reinstate Monica
    Commented Feb 9, 2018 at 19:13

1 Answer 1

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According to the Manufacturer's documentation, the common wire from the transformer connects to the board at pin 6 of the Molex connector.

ICM280 Molex Plug Pin Out

According to the wiring diagram, there should be a junction off the transformer, where one wire goes to the Molex connector, and another goes to the ground (chassis). This junction may be directly at the transformer terminals. This would likely be the best place to connect your new C wire.

ICM280 Wiring Diagram

Make sure you use a voltmeter, to verify that those are indeed the correct pins.

If there's not an extra wire in the cable that runs up to the thermostat. Then you'll have to pull a new wire/cable.

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