If you’re researching online to buy a new thermostat, you may have stumbled upon the term “C wire” or “Common wire” many times. Just like many other folks, you might be confused “What is a c wire?” In this guide on “thermostat c wire” – we explain everything you need to know about the Common wire,  known popularly as the C wire.

The purpose of a C wire is to ensure continuous flow of 24 volts AC power supply to the thermostat.

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.

Note: You might be thinking the R wire (Red wire) from furnace is already a power line and why a C wire is required! Well, the R wire is a power source but it does not ensure a continuous supply when the furnace is OFF. The C wire is more like a neutral wire (in electrical terms) and it completes the electrical circuit between the furnace (or any heating system) and the thermostat, thus ensuring a flawless power supply to thermostat even when the furnace is OFF.

How to know if you’ve C Wire

To know if you already have a C wire in your home wiring, there are two ways.

  1. Look at the backside of your thermostat
  2. Look at the circuit board of your furnace (heating system)

Examine 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

In some cases, your old thermostat may not have a ‘C’ terminal. In such cases, there are chances that a ‘C wire’ is already in place but left unconnected. Look for any wire that is tucked away behind your thermostat. If there is any such wire, verify it as the common wire by examining the circuit board inside your furnace.

Examine Circuit Board of your Furnace/Heating System

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

I hope that was very clear! You now know how to look for a c wire or common wire in your heating system wiring. If you already have a C wire in place, you just need to connect the same C wire to the new thermostat (when you replace the old model). It is very simple and pretty straight forward. Read the installation manual of the new thermostat you buy and wire the device accordingly.

What if you don’t have Common Wire

If you don’t have a C wire running from the furnace, you’ve the following options.

  1. Install a new C wire
  2. Use a C wire Adapter
  3. Choose a thermostat that doesn’t need C wire or has an alternative solution

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