I'm used to the gas furnaces, and I still want to swap out this 4 year old heat pump, but it would be too expensive. I recently bought a smart thermostat to try and help with the utility bills, but it requires a "C" or common wire. The thermostat did include a "C wire adapter" but it assumes that it is being used installed in a gas furnace with a control board. While looking at the system I noticed that it doesn't specify/label the 24v side or the 110/220 side. And as I REALLY do not want to experience another 220 flash. Can someone point me in the right direction?

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  • Can you post photos of the actual thermostat wiring at the air handler please? Dec 22, 2020 at 12:39

1 Answer 1


You don't have the C wire brought out to the thermostat.

See that COND UNIT CONTACTOR On the right is the Y wire going to the stat On the left is the C wire going to the transformer common. It is not labeled C but nevertheless it IS the C wire.

If you have more than 4 individual wires going to the stat then you would have an unused wire, you could connect that to the left terminal of the COND UNIT CONTACTOR and get your C wire brought out to the stat

Or you could run new stat wiring.

Or you could wire in the C wire adapter. Usually how those work is they take some of the signaling wires like G, W or Y wires, convert them into a multiplexed set on a single wire, then the thermostat breaks out the multiplexing internally. Then you repurpose the now-unused G W or Y wire to a C wire.

By the way if you live in a temperate climate then the heat pump isn't worse than a gas furnace. I'm like you, I am old-school with a central gas furnace and a central air conditioner. But I'm in a temperate climate and a lot of people around here run heat pumps and their costs are about equivalent to my combined electrical/gas.

Where heat pumps really suck (in my opinion) is cooler climates where you get a lot of snow and such in the winter and it's not that hot in the summer. Such as like Montana 50 years ago before Global Warming scotched everything.

  • See that COND UNIT CONTACTOR Actually, the image says "CONTRACTOR". But I am 99+% certain that CONTACTOR, as you spelled it in your answer, is correct, and that the manufacturer got it wrong. Could be that the engineers handed the drawing off to a graphic designer to make it look nice, and that designer had no idea what a CONTACTOR was and changed it to CONTRACTOR because "CONTRACTORS install these things." Jan 21, 2021 at 14:57

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