What To Know About Thermostat C Wire

When installing any thermostat, the first important thing that you should take care is the c wire. You need to know that c wire can offer power to the thermostat. To the old models, they were often dialing, and they didn’t need to consume a huge power. Meanwhile, the modern thermostats with Wifi or backlit display require a stable supply. Therefore, the thermostat c wire (also called common wire) allows the constant flow of 24 VAC power to the thermostat.

In a technical way, the power runs from the R (red) wire, but not progressively (not on its own). So, in order to make it progressive, you need to have a common wire to fulfill the circuit. Of course, when the circuit is accomplished, 24V energy will constantly flow. In case that you are going to buy a wifi thermostat, you can install it by yourself. If you used to change a light switch or container, you could perform it – expecting your system has already a C wire.

In case that your system is equipped with a c-wire, it could be in use or just hidden behind your current thermostat. In case that your system doesn’t have the common wire, you have to operate the new cable from your furnace to the thermostat to install most of the modern thermostat models. The thermostat wire normally comes in various weights. In case that you have an HVAC, you should prepare 18/5. In case that you just have the heat, 18/3 might be sufficient, but the best way is to operate 18/5 for a bit of future-proofing.

Two different ways to recognize your thermostat’s wires

# Way 1 – Look at the wires behind the thermostat

C wire thermostat

Disconnect the thermostat from the wall and observe the wires connecting to it. If you have got a wire connected to the terminal labeled “C”, it will be okay to go with the smart thermostat installation. In case that you don’t see a c-wire at the thermostat, don’t think that you don’t have one just yet. It could be loaded into the wall because some installers think that it’s not necessary to show the c-wire.

# Way 2 – Look at the wires inside the furnace

Detach the power to the furnace and pop the cover off. Then observe a row of screws marked R, C, W2, G, Y/Y2…

thermostat c wire

With this picture, this furnace is not equipped with a c-wire hooked up. So, in order to make this HVAC system appropriate to the thermostat, it would need either:

  • The Venstar Add-a-wire or
  • The smart thermostat (i.e. Ecobee3) that comes with a Power Extender Kit for system without a c-wire
  • A new bunch of wires goes to the thermostat and furnace

In case that poking around the furnace or thermostat is not simple, you should consider hiring a professional to install the thermostat.

Remember that there is no rule for thermostat wire color because any wire can be availed for any aim. A previous helper might have been “creative”, so what you see behind the thermostat could change from what you read on the internet. Thus, I will show you some common uses of wire colors, as:

  • Yellow – Y – Air conditioner
  • Green – G – Fan
  • White – W – Heat
  • Black/Blue – C – Common wire, might be unused by your existing thermostat. Permit continuous power run from the red wire.
  • Red – R – 24VAC from the furnace.
  • Red – Rc – 24VAC (specialize in cooling call)
  • Red – Rc – 24VAC (specialize in heat call)


  1. Vicky says

    I am assuming this is a test quoitsen and the 3 lines are the available answers? In which case there should be another answer, none of the above. Because the hot wires can never be green or white.IN most cases they are only covered in black insulation unless it is a 60 amp service which is rarely put in a new building of today. They are then phase taped to indicate the phase and voltage.

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