I am trying to install a new Ecobee3 wi-fi thermostat to replace my old mercury thermostat. When I pull off old thermostat there are only 4 wires (red, white, yellow, green and a blue tied around). When I look at my furnace there are same 4 wires, plus two additional red and white wires (white is occupying com 24v and red is on Y terminal). Those two wires appear to be running outside to my A/C unit. I do have power extender kit that Ecobee supplies, but unsure which wire goes where or how to fix easily since there is no C wire running to thermostat. thanks for your help.wall image[![furnace image]2

  • Is that blue wire on the thermostat cable just tucked in/back at the thermostat end as well? It appears you have a spare wire for your C wire right there... Commented Jan 14, 2017 at 16:58
  • In fact, can you post a photo of the wiring at the thermostat end? Commented Jan 14, 2017 at 16:59

1 Answer 1


Connect the blue wire from the thermostat cable, to the C terminal in the furnace. Then connect the blue with at the thermostat, to the C terminal of the new thermostat.

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