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Old thermostat was only using 3 wires, connected to W, G and R-Rc jumper. no C wire. Does have extra wires not being used at each end. Big question is what to I connect to at the furnace for a power source for the new wifi thermostat? There is only Y, and Com 24V Terminals available on the furnace board. Can I use Com 24v at furnace terminal and C wire at thermostat?

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    Can you post a photo of each end? – ThreePhaseEel Nov 3 '16 at 2:52
  • What's the make and model of the furnace? – Tester101 Nov 3 '16 at 15:32
  • Do you have a volt/multimeter? If so, what's the voltage between the R and COM 24V terminals? Is there currently anything attached to the COM 24V terminal? – Tester101 Nov 3 '16 at 15:33

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