How to Add a C-Wire to Your Thermostat: An In-Depth Guide

Disclaimer: Contractors.com is not receiving any type of compensation for reviewing any of the products or services mentioned in this article.

You’ve probably heard of terms like “low voltage” or “24v” while researching upgrades for your thermostat. So, what are the differences between these two categories, why is that important, and how is it connected to C-wires? 

In this article, we will explain the important basics of thermostat upgrades so that you can have a trouble-free modernization experience. And if you want to increase the functionality of your thermostat, a C-wire will be the best way of doing so. 

Let’s begin by exploring what a C-wire is and why it’s so beneficial to add to your thermostat. 

What Is a C-Wire?

Thermostat C-wires provide a constant current, perfect for smart Wi-Fi enabled thermostat units.

Also known as a common wire, the C-wire is an additional circuit that works with a 24-volt thermostat system and provides it with a continuous flow of electricity. This wire runs from the furnace into the thermostat, and its purpose is to enable the installation of a smart thermostat. 

Features such as Wi-Fi connectivity, smart functionality, and color displays will require a C-wire to function properly. A regular thermostat only turns on when it is needed and then goes into a standby function. While this is fine for your standard thermostat, it will not work for smart devices, and so a circuit that does not have the common wire will have to be upgraded. 

Does Your Thermostat Have a C-Wire?

If you have five or more wires and a black wire as displayed in this picture, you have a 24-volt C-wire equipped system.

Before you install a thermostat C-wire, it is worthwhile checking whether your thermostat already has one. More and more new thermostats come with this wire already installed as manufacturers want to make converting to a smart thermostat an easier process. Verifying whether your thermostat has the C-wire already is a simple two-step process. First off it’s a good idea to make sure that your thermostat is indeed a 24-volt one. 

Check for a 24V Low Voltage System 

24-volt thermostats are the most common type out there, but there are others as well. Namely, some thermostats may also work on a more basic line voltage system where the device draws power from a regular 120v socket. This type of thermostat will not be compatible with C-wires. 

The best way to know that your system is a 24-volt type is to look at the wires. If you have two to nine different multicolored thermostat wires running out of your furnace, then you likely have a 24-volt system. Most central air conditioning systems, or systems that connect wires to a boiler or heat pump, will also be 24 volts. 

However, if your thermostat has between two to four thick thermostat wires and your heating system is decentralized, meaning you have a separate heater for each room, you are most probably looking at a 120v system.  

Check to See if You Have a Hidden C-Wire

If you’re having trouble finding the C-wire, you may need to look behind the plastic frame to see if it was hidden in the wall.

Now that you know your thermostat is a 24-volt model, it’s time to remove it and see the wires beneath it. Most thermostats are easily removable by hand, allowing you to see the wires and how they are hooked up. 

Unfortunately, thermostat wiring is not standardized and so wire colors can vary for different manufacturers. A blue wire can be the thermostat’s C-wire, but this is not always the case. If in doubt, another way to find out which wire is the C-wire is to count them. If you find five wires, and then a sixth black one, that means you have everything you need. 

If after all these checks you still don’t find a C-wire, the next step is to remove the plastic frame that secures the thermostat to the wall and peer behind it. Sometimes a thermostat’s C-wire is not connected but rather left rolled up inside the wall. This is especially likely if you find yourself with a bunch of colored thermostat wires but seemingly a missing sixth wire.  

If You Have a C-Wire and a Low Voltage System 

Once you have a C-wire, installing your upgraded thermostat is a simple matter.

If you find the C-wire, congratulations! This means your thermostat was installed already equipped with everything you need to add a smart thermostat, and that makes your job so much easier. The icing on the cake is that now all you have to do is find a smart thermostat that is to your liking, and there is a multitude to choose from, each with a host of amazing practical features. 

If You Don’t Have a C-Wire 

Some smart thermostat units will come with their own adapter, meaning you don’t have to install a C-wire.

If you don’t have a C-wire, this means you have a few more steps before you can get your smart thermostat system. Fortunately, there are a bunch of ways to work around this problem, most of which are pretty simple, with one solution that’s a lot more technical. Assuming you have a 24-volt low-voltage system, here are some things you can do to get around not having a C-wire. 

Add a C-Wire Adapter

If your system has four wires and the fifth is missing, you’re a good candidate for a wire adapter. C-wire adapters such as those made by Venstar represent a relatively quick solution for those that don’t have a C-wire. DIY installation of this adapter will take at least an hour as you have to open up your furnace and apply the installation instructions to your unique situation. 

You also need to have a minimum of four thermostat wires for the C-wire adapter to work properly. This is largely a plug-and-play add-on though, so once it is installed properly it should require no further attention. 

Add a C-Wire Transformer 

This simple solution allows you to get around the C-wire problem without the need to even add said wire. Instead, a C-wire transformer plugs directly into a household electrical outlet and is connected to your thermostat. Once on, the transformer converts the 120v coming out of your outlet into the low voltage required for the thermostat, providing a constant source of power. 

As long as you have an outlet within twenty feet of the thermostat, all you have to do is wire the transformer up to the thermostat by connecting one wire to the C terminal and the other wire to the Rc terminal of the thermostat, then you just plug the transformer into a wall socket and it does the rest. 

Upgrade Your Thermostat 

Some simpler Wi-Fi thermostats are designed to work without the need for a C-wire.

Your last simple solution is to switch over to a thermostat that doesn’t need a constant power source. If you don’t feel like installing a C-wire adapter or going through the trouble of installing a new wire, this is the next best thing as there are some smart thermostats out there that do not require a thermostat C-wire to work properly. 

Namely, systems such as the Lux Geo Wi-Fi Thermostat can use either two wires or AA lithium batteries to get all the power they need. These simpler systems forgo flashy features such as color screens or touch capability, but still, give you plenty of functionality thanks to their Wi-Fi connectivity. 

Install a C-wire 

The most challenging option is to put in a new thermostat wire. This requires a lot of work because you will have to remove the current wire that runs between your thermostat and furnace and put in a new cable with 5 wires or greater. Depending on your home, this may mean running the wire over a great distance through walls and floors, and as you can imagine, this will require a lot more work. 

We recommend only doing this if you are a well-seasoned DIYer or if you are going to hire a certified electrician to help you out. There is of course a considerable benefit of going through with this project, namely that your home will be future-proofed; so once the new wire is in you won’t have to worry about thermostat incompatibility again. 

If You Have a Line Voltage System

If after checking your thermostat you determine that it is based on a line voltage system, your options are unfortunately limited. Smart thermostats are by and large not compatible with these systems due to how a line voltage circuit is wired. However, you still have a couple of workarounds. 

Get an Adapter 

There are adapters on the market designed to step down the 120 volts of a line voltage system to the 24 volts of a low voltage system. Once this adapter is installed, you will be able to get any smart thermostat you want for your electric wall heater. 

Having said that, these adapters are bulky, so it would be best to find a way to bury them in the wall so that they do not become an eyesore. It may be best to get an electrician to help you out with the installation. Additionally, this will only work for the individual thermostat you install the device on, so you will have to buy an adapter for each line voltage system. 

Find a Wi-Fi Line Voltage Thermostat

While there are not many line voltage-compatible smart thermostat models out there, there are a few. A thermostat manufacturer called Caleo manufacturer carries Wi-Fi enabled thermostats for line voltage systems; meaning they can be controlled remotely via a computer or smartphone just like other smart thermostats. Installation is easy, although these systems only work with baseboard heaters that do not have a fan or motor.