The C-Wire, or “common wire” enables the continuous flow of 24 VAC power to the thermostat. It’s a wire that (if you have one) runs all the way from your furnace to your thermostat.
Questions about the C-wire are by far the biggest drivers of traffic to this site, so I’ve written this article to help you understand what the C-wire is, help you find your system’s C-wire, and help you decide what thermostat to buy based on what wires you have and/or are willing to install.
To find out if your system has a C-wire
The simplest way is to pop your thermostat off the wall and look at the wires behind it and where they are hooked up. Remember, the colors don’t really mean anything. Think of the colors as a suggestion: the C-wire is usually the blue wire, but that’s not a standard.
Best case scenario: you find five wires (white, red, yellow, green, blue) and possibly a sixth black wire. Everything you need is here!
Almost there scenario: you find three or four thinner multicolored wires.
If you don’t see a wire you can identify as the C-wire, though, don’t despair yet – you may need to look behind your existing thermostat. If your current thermostat didn’t need a C-wire, it (or a wire that can be used as a C-wire) might be rolled up inside the wall. This is more likely to be true if you see all the other color wires present.
Line voltage thermsotat scenario: you have just two wires (white and red, probably), and they’re thick. You most likely have a line voltage system and none of the popular smart thermostats on the market are available to you as-is, but there’s hope with an adapter.
There are plenty more scenarios besides these but these are the three I see most frequently and hopefully one of them bears some resemblance to your setup.
As an aside, a lot of manufacturers assume you have both heating and cooling. If you just have heating, you might find you use fewer wires than they assume you need to.
Is your system line voltage or low voltage (24V)?
This part matters – the best known smart thermostats (Ecobee, Nest, Honeywell) are for low voltage systems (with or without a C-wire, depending on the thermostat you choose).
CadetHeat has an excellent article on line voltage vs. low voltage and how to tell which you have.
You probably have a low voltage system if:
- Your wires are thinner (and multi-colored)
- You have a furnace, central air conditioning, boiler, heat pump, or split system
- You have central heating
- Statistically speaking, low voltage systems are more common in the United States
You might have a line voltage system if:
- Your wires are thicker and either 2 or 4 in number (probably just two or four, red and black in color)
- Your heating system is not central heating with a furnace and ducts
- Your thermostat(s) control just one heater apiece
- You operate your heater(s) on a room-by-room basis
- Your heating system is a radiant, convection, or resistance heater
- Your place is a cabin, ski lodge, cottage, small apartment, or city condo
Yay, I have a low voltage system and a C-wire!
Lucky you! Take your pick of the best smart thermostats on the market today. (Now, if you have a heat pump or a boiler or other accessories you should check for compatibility on those, too, but chances are if you made it this far, there’s something on the market compatible with your system.)
Help, I don’t have a C-wire!
If you don’t have a C-wire and your heart is set on having a smart thermostat in your home, your have a bunch of options (which can be mostly be boiled down to “get a C-wire” or “get a smart thermostat that doesn’t require a C-wire”).
Here are those options, starting from “most involved” and going all the way to “I hate this stuff, give me something easy”.
These first few options assume you have a low voltage system.
Option 1: Run new thermostat wire (the 5 conductor kind or greater)
This is the nuclear option. You’ll have to pull out the existing thermostat wire that runs between your furnace and your thermostat and run new wire through the floors/walls. Depending on your home, this may be a short distance or daunting task. You might want to hire a professional to do the job.
On the bright side, this option future-proofs your HVAC wiring and will let you choose basically any smart thermostat you want in the foreseeable future.
Option 2: Get a Venstar Add-a-Wire adapter
If you already have four wires and just need a fifth to be the C-wire, you’re a good candidate for a Venstar Add-a-Wire adapter [our review].
Adding a C-wire to 24VAC systems is what this thing exists for. The only catch is you have to already have a minimum of four wires present. Oh, and you will have to open up your furnace and probably spend 20-40 mins opening things up, applying Venstar’s instructions to your unique situation, and hooking it up. It should be straightforward, though.
Option 3: Choose a thermostat that includes an adapter
Smart thermostat manufacturer ecobee has a great solution to the missing C-wire problem: they include a “power extender kit” with two of their thermostats. The Power Extender Kit is easy to hook up, and the ecobees are great thermostats.
The ecobee3 Smart Thermostat [our review] has been our top-tier best smart thermostat pick for over a year now and the ecobee Smart Si [our review] is one of our favorite budget smart thermostat choices.
Buying an ecobee thermostat is a great option if you are missing a C-wire, especially if the ecobee3 is what you want anyway (and you should, it’s great). If you’d rather get a different smart thermostat, though, then you should go with the Venstar Add-a-Wire (option 2 above) and pair it with the thermostat you want.
Option 4: Choose a thermostat that doesn’t require a C-wire or an adapter
If you have a low-voltage system with too few wires and you don’t want to (or can’t) run new wires, then check out the LuxGeo: many of its users report it works in their HVAC system when nothing else does.
The Lux Geo WiFi Thermostat [our review] is my favorite recommendation to homeowners whose existing thermostat wiring is just completely incompatible with the market’s current popular smart thermostats. The LuxGeo has the option to run on AA lithium batteries and is suitable for 2-wire systems. It’s also a fantastic thermostat besides all that.
Wait, I have a line voltage. Can I get a WiFi thermostat for my line voltage system?
Line voltage option 1: Add an adapter
If you have a line voltage system you may still be able to get a mainstream smart thermostat if you use an adapter to step the voltage down, such as the Aube RC840T-240 [Amazon.com].
With this adapter in place, you can shop for the low voltage thermostat of your choice and use it with your electric wall heater, electric baseboard heater, radiant heat, etc.
Line voltage option 2: get a thermostat designed for line voltage system
As of this writing in early 2016 there are not many WiFi-enabled options for line voltage thermostats.
If you have a baseboard heating system you’re in luck: Caleo by CaSA is a WiFi enabled thermostat [Amazon.com] designed for use with electric baseboard heaters. It’s IFTTT compatible and you control it via your computer or your smartphone by logging into Caleo’s web-based portal (not an app, just a website, but it’s a mobile friendly website).
Note: Caleo doesn’t work with forced air heaters, so if your thermostat controls a heater/fan in your wall, this isn’t the thermostat for you. In that case, you should look at the adapter from the previous option.
Whew – and that’s the basics. There’s a lot of variation among heating/cooling systems and chances are, your heating system is different than what is featured here. In all cases, the best first step is to figure out what heating system you have and work from there: find out which manufacturers support your system and accessories. Most, if not all, smart thermostat manufacturers have a help line you can call for personalized support, too, so give ’em a call if you’re truly stumped.
Remember, this is a site run by tech enthusiasts, not an HVAC expert familiar with your system, so be careful when you mess around with wires!