The “C” wire, also known as the common wire, is used to provide a continuous source of power to the thermostat in HVAC (Heating, Ventilation, and Air Conditioning) systems. The exact location to connect the C wire on a furnace is the terminal labeled “C” or “COM” (common) on its terminal strip. This is often found on the control board of the furnace.

How to find C wire terminal on the furnace

In USA, the most commonly used heating systems are gas furnace, boiler or a heat pump. Some households use a combination of gas furnace & heat pump. In any case, your heating system will have a circuit board with terminals marked similar to the thermostat like R, W, Y C, O/B etc. You can identify/verify the common wire by examining this circuit board. Take a look at the 2 images given below.

Furnace Circuit Board - C Wire
C wire from furnace circuit board going to thermostat – source

If you observe the terminals, you can see two wires are tapped from ‘C’ terminal. The ‘black’ color wire is part of the set that goes to the thermostat. The ‘white’ color wire is part of the set that goes to the condenser unit. In another image given below, you can see the same C wire going to the thermostat terminals side.

Furnace common wire or c wire
C wire from furnace to thermostat – Source

What if there is no C-wire to the furnace

In some cases, you may not find a c-wire to connect to the furnace. In this case, check if there is a C-wire connection coming from the thermostat.

Examine the Backside of your Current Thermostat

The easiest way to know if your home wiring already has a common wire is to examine the backside of your current thermostat. To do this examination, you may remove the face plate and take the thermostat out of the wall. Now you will see wires coming into different terminals of your thermostat marked R, W, Y, G, Rc, C, O/B etc. Look specifically for a wire attached to the terminal marked ‘C’ (as shown in the image below).

Caution: Before removing the face plate of thermostat, make sure you’ve cut the power supply to the thermostat (and the furnace/heating system).

Common Wire - Thermostat
“C Wire” coming into thermostat – Look “Blue” wire in ‘C’ terminal – Source

In the above image, a “Blue” coloured wire is coming into terminal marked “C”. Usually “C wire” or the Common Wire will be a blue coloured wire. But there is no particular standard for thermostat wires, so it can be any color like Yellow, Brown, Green or any other color.

If you see a wire of different color, say Yellow in the ‘C’ terminal of your current thermostat, most probably it should be your ‘C’ wire. You can verify this by opening the circuit board of your furnace, and make sure the same color wire is connected to the ‘C’ terminal on the furnace circuit board.  Take a look at the image given below.

C Wire in Thermostat
Yellow color wire connected in ‘C’ terminal – Source

7 Steps to connect C Wire to a Furnace

Connecting a C wire to a furnace involves a few steps. Here’s a general guide on how to do it:

  1. Turn off the power: Before working on the furnace, ensure that the power is turned off. Locate the power switch or circuit breaker that controls the furnace and switch it off to avoid any electrical accidents.
  2. Locate the C terminal on the furnace: Open the furnace cover to access the control board or terminal strip. Look for a terminal labeled “C” or “COM” (common). It is often color-coded as blue or black.
  3. Prepare the C wire: If you don’t have a C wire already, you may need to run a new wire from the furnace to the thermostat. The C wire should be connected to the C terminal on the furnace and the C terminal on the thermostat. Ensure the wire is long enough to reach both points comfortably.
  4. Connect the C wire to the furnace: Strip about 1/2 inch (1.27 cm) of insulation from the ends of the C wire. Insert the exposed wire into the C terminal on the furnace. Depending on the type of terminal, you may need to use a screwdriver to secure the wire or push it into a clamp.
  5. Secure the connection: Once the C wire is in place, make sure it is securely fastened to prevent any loose connections. This will help ensure a stable power supply to the thermostat.
  6. Double-check connections: Take a moment to double-check all the wire connections, ensuring they are secure and properly attached. Make sure no other wires are loose or disconnected.
  7. Restore power: After confirming the connections, close the furnace cover and restore power to the furnace by turning on the power switch or flipping the circuit breaker back on.

Why do thermostats need C wire

Back in the olden days, thermostats were just on/off switching devices and they used to draw the necessary power from the heating system itself (be it a furnace or a boiler). There was no need for the thermostat (old models) to be turned on 24/7, as they had nothing much to do while the furnace is not working.

Modern smart thermostats and WiFi enabled thermostats are feature rich, and they do continuous monitoring of weather, closely watch furnace performance (for energy savings), they have voice control, gps based features like geofencing, and they can be controlled from away places via internet. All these features demand the thermostat to be functioning 24 hours a day, 7 days a week, all the year. Thus modern thermostats require a continuous power supply and the C wire or Common wire is required for most thermostats to ensure 24 volts AC supply continuously.


I hope that was very clear! You now know how to look for a c wire or common wire in your heating system wiring, and how to connect it to the furnace.


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