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I'm trying to replace my old mercury thermostat with Honeywell Rth8580wf. The cable has 8 wires, only 4 were being used. The wire colors and letters on the thermostat do not match up, so I'm left with red, white, yellow and green being unused. I tried to connect the red wire to terminal C, but the thermostat did not power up. I thought I could use any unused wire as a C wire, but please correct me if I'm wrong. If I can in fact use any wire, why doesn't it power up? I wish I could attach pictures, but can send upon request.

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You can't simply connect any extra wire to the C terminal on the thermostat. You have to also connect the other end of that wire to the C terminal in the furnace.

If you include a photo of the wiring at the thermostat, and a photo of the wiring in the furnace I can give you a detailed list of instructions for hooking it up.

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If you noticed, the new thermostat came with detailed instructions and labels for the wires. The new one should have been wired exactly like the old one. IE: R/RW/RC-W-G-C-Y, etc.

Without knowing how the old one was wired it is impossible to say.

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