What Is C-Wire on a Honeywell Thermostat? (You NEED to Know This!)

If you have a thermostat, you probably hear the word c-wire a lot. You probably already know all the components of your smart thermostat, and a c-wire isn’t one of them. So what is a c-wire on a thermostat? And do all thermostats need a c-wire? 

A c-wire is a cable connecting your heating system to your thermostat. Its main job is to provide your thermostat with continuous power, allowing it to carry out its advanced functions without shutting off.


Read on to find everything you need to know about the matter!

What Is C-Wire on a Honeywell Thermostat?

C-wire is an abbreviation for common wire. Essentially, it’s an extra cable that creates a return path for continuous power. It supposedly works for any application, but only a few applications demand one like some thermostats do.

In a Honeywell thermostat, a c-wire connects your thermostat to your heating and cooling system, providing the thermostat with continuous power. Back in the day, thermostats didn’t require c-wires, but they didn’t have half of the functions that these devices have today.

To power the Wi-Fi functions, LED displays, and all the advanced features, the thermostat needs the c-wire to keep providing power. So, if you plan on getting a smart thermostat, it’s better to make sure you have a c-wire.

Do All Thermostats Require a C-Wire?

Technically, having a c-wire is better because it guarantees your heating system will be compatible with your thermostat, regardless of its brand and model. However, not all thermostats require a c-wire.

For example, most Honeywell thermostats demand a c-wire, but some Google Nest thermostats don’t. So, it depends on the thermostat you’re buying and its functions. There are alternative ways as well, as outlined in the video below:

Why Are C-Wires Important for Thermostats?

If you’re planning on installing a smart thermostat anytime soon, you may find people advising you to make sure you have a c-wire first. But why are c-wires important for thermostats? 

Well, for one, today’s thermostats need more and more power. In the past, you didn’t need a lot of power to operate a thermostat because they weren’t wireless, and you didn’t have to maintain a Wi-Fi connection that eats up the battery.

Now, the thermostats need to receive enough power to keep powering the Wi-Fi connection and advanced functions they come with. Unfortunately, no AA battery will ever be enough for that.

How to Know If Your Thermostat Uses a C-Wire

If you’ve had your thermostat for a long time now and you want to know if it runs on a c-wire, follow these steps to know for sure.

  1. Turn off Your Heating System

The first step you should do is turn off the heating system, so you can check the thermostat without the risk of damaging it or overheating the wires. You can easily do so through the breaker box.

Next, turn off the thermostat as well. Remember that turning it off alone won’t cut the power from your heating system, so you still have to turn it off separately. 

After that, make sure that both systems are off the power. To do that, adjust the temperature on your thermostat to lower or higher. If the device or the system is on, you should hear the system working to adjust the temperature. If you pass 5–10 minutes and nothing changes, you can ensure the system is off. Also turn the power off in your home and wear proper personal safety equipment.

  1. Remove the Thermostat

The next step is removing your thermostat from the wall. More often than not, removing the device will be as easy as pulling it from its outlet. It should come out easily, then. However, not all thermostats are made equal. Some of them have screws, which will need more effort to pull out.

When removing the thermostat, make sure not to pull any wires.

  1. Look for the C-Wire

Now that you have the thermostat off the wall, look at the wiring behind it. There should be a terminal labeled C; if there’s a wire connected to it, then you have a c-wire. If there’s no wire in the terminal, then your thermostat is running without a c-wire.

How to Identify the C-Wire on Your Thermostat

You can easily find the c-wire on your thermostat by popping the cover off and looking behind it. You should see five wires in different colors connected to your thermostat. If you have a c-wire, it’ll be connected to the terminal labeled C on the device. It also often comes in blue color, so you can identify it even if the label isn’t there.

It’s worth noting, though, that common wires can come in any color. Blue is the more common color, but it’s not a standard. So, don’t rely completely on the wire’s color when identifying it.

Not only that, but the five wires are also not a standard. You may find fewer wires, which in some cases means that you don’t have a c-wire. Or, the wires may not be visible because they’re tucked into the wall. In this case, you may need professional help to identify the wires.

Can You Add a C-Wire to an Already-Existing Thermostat?

If you find no c-wire on your thermostat, you may be tempted to install one to provide continuous power for the device. In this case, you can easily do it using an add-a-wire kit you can buy at any market.

The kit will allow you to add a c-wire without stealing any power. All you have to do is install it, which is done at the HVAC board. Before you attempt to install the kit, you’ll have to locate the furnace’s panel, then turn the power off to avoid damaging the board while working. 

If you can’t install it yourself, you may want to get professional help because you’ll need to be able to read wiring diagrams to do it. Plus, a control board is expensive, so you don’t want to mess around with it. 

Honeywell is the market leader when it comes to thermostats. They have a lot of models. More recently, they have started to offer advanced digital models. These can require a c-wire or other methods to install them.

It is easy to install a Honeywell thermostat as they have great manuals and there are a lot of videos online. They also offer support online and you can even call them if you have a problem. Plumbers work on thermostats but they generally need the help of an electrician to get them working.

Closing Thoughts

C-wire is the abbreviation of common wire, and it serves to create a return path for continuous power. That way, your heating or cooling system keeps powering the thermostat, allowing it to keep doing its advanced functions without shutting off.