What Is a C Wire for a Thermostat?

Remember the thermostat in the home you grew up in? It probably looked a lot different than the thermostats you see today, which have different needs than those from even just a couple of years ago.

One of those needs is a continuous power source. A "C" wire, or "common" wire, supplies power to a modern thermostat. Today’s thermostats require a C wire because they need constant connection to a power source to run multiple systems and maintain an internet connection.

Read on to learn more about the importance of a C wire.

C Wires Explained

An important thing to keep in mind about C wires is that they are not necessarily going to be any particular color nor labeled as a C wire. C wires can vary across the board, but they’re called “common” for a reason: most Wi-Fi thermostats need to have one.

Where to Find Your C Wire

Before you start poking around the wiring of your thermostat, turn off the power! Each thermostat is different, so consult your instruction manual if you don’t know how to power down your thermostat for safe wiring work. When it’s safe to explore, start by looking for terminal labels.

Each wire in the back of your thermostat connects to its corresponding terminal. For example, the C wire connects to a specific C terminal. Each terminal is responsible for controlling a different aspect of temperature control.

Check for a terminal labeled “C.” If there’s a wire connected to that terminal, then you have a C wire. If the C terminal is empty, that probably means you don’t have a C wire.

There are, however, a couple more things to look for before calling it quits:

1. Loose wires. You may have a C wire that is not hooked up to your older-model thermostat.
2. Your HVAC control board. You may have a C wire attached to the C terminal of your control board. If there is a wire there, note what color it is. Your thermostat C wire will match the color of the C wire you saw in your control board.

What If My C Wire Is Missing?

Don’t try to run your modern thermostat without a C wire. It’s important to have a C wire to ensure your thermostat is always connected to a power source. The simplest solution to accommodate your new thermostat upgrade is to add a C wire to your thermostat wiring. Because this will involve getting into your HVAC control board as well as the wiring of your thermostat, it’s best to call a professional to install your C wire for you.

Now You “C” It …

Your local Aire Serv^® is here to help with all your HVAC needs. We can check the wiring of your thermostat for a C wire and, if you don’t have one, we can install one for you. At Aire Serv, we prioritize quality work. Call or request an appointment online to get started.