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What is a C-Wire for a Thermostat?

As people continue to swap out their traditional household appliances for 'smart' versions, it's important to know how to go about integrating smart technology into your home safely and properly.

While there are many different smart thermostats on the market today, this post is going to focus on the use of C-Wires, why they have been incorporated into the use of this smart device and how to make sure you are choosing a thermostat that is compatible with your home.

Each answer attempts to offer a well-rounded approach to understanding C-Wires and why they are important with smart thermostats, so that whether you do, or don't have one, you can know how to move forward with a smart thermostat that will actually work for your home.

So what is a C-Wire and how do you know which smart thermostat is right for you?

To help you decide which thermostat would work best in your home, I've put together a list of questions and answers around C-Wires and how they relate to thermostats.

In this post, I discuss what a C-Wire is, how to know if you have one, what to do if you don't, and smart thermostats you can use without one.

By the end of this article, you should have a good idea of what a C-Wire is, whether or not you have one and what kind of smart thermostat you can integrate into your home based on that information.

Let's jump in.

So, what is a C-Wire for a thermostat?

A C-Wire, also called a "common wire", is an electrical wire that runs from a low voltage heating system and carries continuous power to your thermostat. Most newer heating and cooling systems have C-Wires today and will be found in most newly built homes. This is fairly standard in today's electric market. This would guarantee compatibility of any smart thermostat in your home.

How do I know if I have a C-Wire?

It is pretty easy to see if you already have a C-Wire connected to your system. All you have to do is remove your current thermostat's face-plate from its base-plate and look for the terminal labelled with the letter "c". If this terminal has a wire attached to it, you have an active C-Wire.

What Color is a C-Wire?

While there is no universal standardization for the color of a C-Wire, you will find that the most common color for a C-Wire is blue.

Is the C-Wire just for power?

A C-Wire is used as a return path to your thermostat, so that there is no disruption to it's power source, allowing it to run features and functionality continuously. This extra wire provides a continuous 24V of power for any application, where other wires are used as electrical on/of switches for your device.

What if I there is no C-Wire for my Thermostat?

If there is no C-Wire, you probably have what is called a Line Voltage System. The difference between these two is primarily in their energy usage. A line voltage system is usually between 120V - 240V and is often used for sockets, plugging in appliances such as coffee pots and vacuum cleaners. This is different from a C-Wire, also known as a low voltage system, that usually operates under 30V, with most smart thermostats operating at 24V.

What smart thermostats can I use without a C-Wire?

You've taken off the face-plate of your thermostat and don't see a C-Wire, but you really like the functionality of a smart thermostat and the idea of saving energy and money while heating and cooling your home. You're in luck! There are some great options out there to accommodate this, without you having to hire an electrician and re-wire your home. Here are our top choices:

1. Ecobee 3 Lite Smart Thermostat

Best Overall Smart Thermostat without C-Wire.

The Ecobee Smart Thermostat comes with a Power Extender Kit. How does that help? It works by connecting to your home's existing thermostat wires to create a complete circuit. This provide the continuous 24V of power that is normally circuited through your C-Wire as a return path. By installing this to your HVAC system's control board, you are allowing it to work like a C-Wire, providing constant power to your Ecobee thermostat.

Add the Ecobee Lite Smart Thermostat to your home today!

Price $179/unit

2. Google Nest Learning Thermostat

Best Energy Saving Smart Thermostat

The Nest Learning Smart Thermostat is best known as the first smart thermostat to get ENERGY STAR certified. It learns what temperature you like and programs itself around your schedule to keep your home at your preferred temperature, while also helping you save money. Energy savings aside, Google has released the Nest Power Connector, which is similar to Ecobee's Power Extender Kit. This is a C-Wire adapter, that helps maintain that 24V flow of power, so that your Nest Learning Thermostat can maintain maximum functionality.

Add the Nest Learning Smart Thermostat to your home and enjoy the benefits of a thermostat that knows your schedule!

Price $249.00/unit

3. Google Nest E Smart Thermostat

Best Basic Smart Thermostat

This smart thermostat looks anything but basic. Turning itself down when you are out of the house, the frosted display shows the indoor temperature and blends in beautifully to any space. You can turn this thermostat on or off from anywhere and use voice control with Amazon Alexa and Google Assistant. We recommend using the Nest Power connector with this thermostat as well, to keep it functioning properly.

Get the Nest E Smart Thermostat and enjoy the beautiful display along with it's awesome functionality.

Price $175.00/unit

Nest Power Connector

Unlike when you purchase the Ecobee, the Nest Power Connector is not included when you purchase your thermostat. Priced at around $30.00 depending on where you purchase it, the power connector is sold separately. It's not a big expense, but something to keep in mind when pricing out your options. Adding smart technology to your space can be expensive upfront, so it's important to know your costs when considering swapping out your devices.

If you're grabbing a Nest Thermostat, you'll want the Power Connector. Gets yours here today!

Price $24.98/unit


When it comes to finding the right smart thermostat for your home, it all comes down to whether or not you have a C-Wire. From there, you can choose the one you think best suits your needs.

While the Nest Thermostats claim to work without a C-Wire, we often find that there are problems in functionality and use when people try to install these thermostat without a Nest Power Connector. Why? In order to make sure there is always enough power to maintain the core temperature control functions, the thermostat will shut down certain features if it is lacking the necessary voltage (power supply) to keep it all up and running. This often negatively impacts the overall functionality of your device. So while each of these smart thermostats is an option you will need some sort of power adapter to provide a constant supply of power, as a C-Wire normally would.

Any smart thermostat is going to reduce your energy consumption and save you money.

Do you have a C-Wire? Whether you do or don't, which smart thermostat would you add to your home and why?

Let me know in the comments below.

Already have the device and looking for installation and setup? Shop our installation services, so you can rest easy knowing your smart devices are being installed and set up properly.

Further reading on Have you ever wondered which smart thermostat on the market today is the best?

By considering the pros and cons from some of the best thermostat currently available on the market, you can get a well integrated smart device that optimizes your energy consumption, saves you money and keeps your house's temperature exactly where you want it, whether you're at home, or away.

Let's be honest, we're all looking for a little more from our devices, so don't forget to check out Thursday's article to see some of the newest and coolest smart thermostat tech options out there.

Knowing what smart thermostat is best for your home will help you leverage the cost savings, while enjoying the cool features and functionalities, so you're not always worrying about your energy consumption and bill.

Disclosure: I may receive affiliate compensation for some of the links below at no cost to you if you decide to purchase a paid plan.

This site is not intending to provide financial advice. This is for entertainment only.

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