As part of the home work before buying a smart thermostat (yes, because of the standard C-Wire issue!), i noticed that my current old thermostat have only four wires (No C-Wire), but my furnace does have one and it is going somewhere else, which i am not able to figure out!

Furnace View 1 (Notice the White from C and Red, shared from Y - marked in Red)

Furnace View 2 (Notice that C and Y taking a different route and the rest to my thermostat)

Thermostat (Only four wires)

Any idea where that C and Y could be going? Not sure if this is related, but I do have a Gas fireplace controlled by electric switches.

  • Do you have an air conditioner? – ThreePhaseEel Jan 8 '17 at 5:07
  • @ThreePhaseEel Yes, i have an AC – James Poulose Jan 8 '17 at 5:14

Air conditioners need a C wire, too!

That separate cable that is connected to your Y and C wires goes out to your air conditioner's outdoor unit. It connects to the coil of a contactor inside the outdoor unit that turns the compressor and condenser fan motors on when you want air conditioning and off when the air conditioning demand has been satisfied.

Heat pumps require a fatter cable as they have more wires going to them, and some modern units substitute a control board for the simple contactor found in an older air conditioner as they have a multispeed or modulating compressor and/or multispeed condenser fan.

  • Thanks for the response. Since i have a C-Wire and that is already used for the AC, does that mean i cannot use it for a smart thermostat (even if i am willing to add a new wire from furnace to thermostat)? – James Poulose Jan 8 '17 at 19:46
  • @JamesPoulose -- no, you can hook up another wire to the existing C terminal and call it a day :) – ThreePhaseEel Jan 8 '17 at 21:56

Yes, the C-wire is used for other stuff, namely the normal loop circuit.

Here are functional (not routing) diagrams of traditional switch loops and thermostat circuits. It's the same basic wiring for any control circuit, be it calls for heat, fan, A/C, aux, heat pump changeover, any circuit. See the similarity?

And below, what happens when you add a smart switch that needs neutral or C. Notice how it's the exact same problem with the exact same solution.

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