how to find out if your old thermostat has a C-Wire, take it off the wall and see how many wires it uses.
Example of line voltage setup for thermostat.

What You Need To Know About Smart Thermostats & C-wires

Here is why and how a C-wire is key to powering a smart thermostat for your new smart home.

Like GearBrain on Facebook

The promise of building a smart home today lies in how easily the devices connect to one another. For instance, it's easy to link your smart thermostat to a smart lighting setup—all with the flip of a switch. However, as we often talk about, building a smart home can be simple if you understand the details. One of the details is a little-known element called a "C-wire," or as it's known in the HVAC (heating, ventilation, and air conditioning) industry, the "common wire."

In learning about smart Wi-Fi thermostats and how they work, we came across this little wire and could not believe how important it was to run a smart home. The C-wire enables the continuous flow of 24 VAC power to your smart thermostat.

Many of today's new smart Wi-Fi thermostats require you to have this little wire to run their connected devices. However, not many homes have a C-wire, causing a lot of frustration among buyers. Why? Not many manufacturers of smart Wi-Fi thermostats inform you of the need for a C-wire to operate their product. Also, when you purchase a smart Wi-Fi thermostat, many salespeople will tell you the device will work without a C-wire. Yet when you install it, you find the thermostat will not work without one.

This happened to me, so believe me, I know your pain if this has happened to you. This is the reason we are writing this article. We are going to help you understand more about the C-wire—and what you need to know before you buy or install a smart Wi-Fi thermostat in your home

Honeywell Thermostat without a C-WireStandard Honeywell Thermostat - Line VoltageGearBrain

Before you even buy a smart Wi-Fi thermostat, determine the type of wiring between your thermostat and HVAC system or your furnace/central air unit. You either take the cover off your existing thermostat to see how many wires are exposed or look at the wires inside your furnace.

Depending on how old the thermostat is, you might see mercury in a glass vial when taking off your thermostat's cover. If you do, you have an old "mercury blob" thermostat, which likely does not have a C-wire. However, to be sure, check the wire connections on your thermostat's front or back. (See images below.) Look inside the junction box to see if you can spot loose cables. Sometimes, a C-wire can be inside your box but not connected to anything.

Old thermostats work without C-Wire.Example of "mercury blob" thermostatGearBrain

After locating the wires, see if you spot only two. If so, it means you have a line voltage connection. Line voltage is the standard voltage found in outlets and junction boxes in your home. The voltage is 120 volts in the US and Canada and is common in older homes. Do you see three wires? The next one might be the C-wire. You can do a few things to determine if one of these three wires is a C-wire. The wire connected to your thermostat's "C" terminal is a C-wire. Some manufacturers use letters like "B" or "X" to label the C-wire or use a color wire, like green or green-yellow striped. If the wires are red and black, note those wires are hot or active and are not C-wires. However, it's important to note there isn't a standard color for wire colors, so be extremely careful handling any of them.

how to find out if your old thermostat has a C-Wire, take it off the wall and see how many wires it uses.Example of line voltage setup for the thermostat.GearBrain

If you decide to look at the wiring inside your furnace, you must first turn off the power to your furnace. Then, pop the cover off and look inside for the row of wires and their connections. The connections should be labeled with letters (like R, B, C, G,…). Depending on your HVAC system type, it could be straightforward to see if you have a C-wire. If you don't have a C-wire, we recommend hiring an HVAC professional to install it. Even if things sound simple, you don't want to mess with your HVAC system.

Once you determine whether you have a C-wire, you are ready to move forward. If you have a C-Wire, you can buy and install smart Wi-Fi Thermostats. (See GearBrain's Best Smart Thermostats for ideas on good smart Wi-Fi thermostats.) If you don't have a C-wire and are determined you have a line voltage system, don't worry. You can still install a smart Wi-Fi thermostat to work in your home.

According to Honeywell, about 40 percent of the homes in the United States have line voltage. Line voltage is largely used as backup heat in most markets, except in Quebec, Canada, and certain regions (like parts of the Northwest). If you have a line voltage system and want a smart thermostat, consider hiring an HVAC professional and have them install a C-wire. Another option is to look for Smart Wi-Fi thermostats that work without a C-wire. Some Wi-Fi thermostats don't need a C-wire because they use power stealing; other smart thermostats offer a device called a wire saver or C-wire adapter. According to Nate Kraft, senior director of Honeywell's Global Home Comfort, there are very few line-volt Wi-Fi solutions in the market today. Honeywell offers one, but "…we recommend a professional installer for high voltage products as a general rule, simply because 120/240V systems can pack a nasty sting," says Kraft. "Honeywell offers a line volt Wi-Fi solution called the eConnect. When eConnect is paired with a RedLINK internet gateway, you now have an app-controlled line voltage system that will work with electric baseboards and other products that use main's voltage." (Update: Resideo, a spin-off from Honeywell that makes smart thermostats and other smart devices, sells a C-wire adapter for you to use to replace your old Honeywell thermostat or any thermostat that has only two wires and connect any new Resideo or Honeywell Home smart thermostat.)

Once you determine the type of HVAC wiring you have in your home—and are ready to get a new thermostat—bear in mind that your heating system may not work with a new smart thermostat. If you have electric or radiant heat in your home, you might have a problem. Today, smart Wi-Fi thermostats do not work with electric heat or certain types of baseboard heating. If you have radiant hot water heat (AKA baseboard hot water heat), it could be powered by line voltage or a low voltage circuit, which " the common power source for radiant hot water heat, but electric heaters (radiant or baseboard) will be line voltage," says Kraft.


Smart Wi-Fi thermostats are a popular addition to any smart home ecosystem. These devices can help make your home run more efficiently and effectively when appropriately installed. If you plan on making home improvements, we recommend upgrading your thermostats simultaneously. You might incur additional costs if you need a C-wire, but the benefits will outweigh the costs in the long run.

Building a smart home today is a commitment to a new, connected life. Don't be afraid of `these new connected devices. GearBrain is here to educate you on the pros and cons of all these new "things" so you can easily embrace your new lifestyle. Maybe one day we won't worry about all this; we'll have robots like the Jetsons did to do all the work for us.

Best Smart Thermomstats - Buyiing GuideGearBrain

Check out The GearBrain, our smart home compatibility find engine, to find, buy, and connect any smart thermostat with your compatible products that work with Google Assistant and Amazon Alexa-enabled devices.

Like GearBrain's content? Be sure to follow us.

Like GearBrain on Facebook
The Conversation (0)

GearBrain Compatibility Find Engine

A pioneering recommendation platform where you can research, discover, buy, and learn how to connect and optimize smart devices.

Join our community! Ask and answer questions about smart devices and save yours in My Gear.

Top Stories

Weekly Deals