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.
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.
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.