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I'm trying to add an amazon smart thermostat. I don't have a C wire, but there is a spare black wire behind the thermostat. I put my multimeter on 200 VAC and expected to see 24 VAC on the red wire. I'm pretty sure I need to plug in my black wire at the furnace to get the 24 V AC common wire. What voltage should I be seeing from the red wire. Shouldn't it have power? What diagnostics should I have?

My furnace is a Lenox G50UH.

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This is my furnace:

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And here is another shot. I can't find where the black wire comes in:

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bonhoffer
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1 Answers1

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I think I've figured it out. Like so many other posts. I had to open the box upstream from the downflow coils. That revealed the control board:

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I'm pretty sure the brown wire in the pic is connected to the thermostat and there is another brown wire below that one that connects to the compressor. My plan is to add the black wire from the first strand to the C terminal at the top that is currently connected to the yellow wire that goes to the compressor. Please add anything if this isn't the right way to go.

I connected the black wire from the large brown wire to the "C" terminal and went back to the thermostat. I found 27 VAC on the red wire but no voltage on the black wire. Maybe the C wire is unpowered.

When I applied power, I got two blinking LEDs.

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I also had this wiring diagram:

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bonhoffer
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