Last updated on September 9th, 2017
Back in the olden days, thermostats were simple on/off devices that didn’t need their own continuous power supply. Modern thermostats with Wi-Fi and backlit display, by contrast, need a steady supply of juice.
The C wire, or “common wire” enables the continuous flow of 24 VAC power to the thermostat.
Technically speaking, power flows from the R (red) wire, but not continuously (not on its own, anyway). To make it continuous requires a common wire to complete the circuit. When the circuit is complete, 24V energy will flow continuously.
If you’re considering purchasing a smart thermostat, you’re probably thinking of doing the installation yourself. After all, if you’re able to change a light switch or receptacle, you’re skilled enough to install a smart thermostat – assuming your system already has a C wire.
If your system has a C-wire, it might be in use or just tucked away behind your current thermostat.
If your system doesn’t have a C-wire, you’ll need to run new cable from your furnace to your thermostat to install most of the modern smart thermostat models.
Thermostat wire comes in various weights. If you have heat and AC, you’ll need 18/5. If you’ve only got heat, 18/3 is sufficient but you may want to run 18/5 anyway for a bit of future-proofing.
Identifying Your Thermostat’s Wires
There are two ways to find out what wires your HVAC system has.
Method 1: Look at the wires behind your thermostat
Detach your thermostat from the wall and look at the wires connecting to it. If you’ve got a wire connected to the terminal labeled “C”, you’re (probably) good to go with a smart thermostat installation.
If you don’t see a C-wire here at your thermostat, don’t assume you don’t have one just yet. It might be stuffed into the wall, which is what some installers do when the C-wire is present but not needed.
You can also look inside your furnace (see method 2). If you find a C-wire in your furnace, then the other end is probably stuffed into the wall behind your thermostat.
Method 2: Look at the wires inside your furnace
Disable power to your furnace and pop the cover off. (The ease of doing it this way will vary by furnace and installation.)
Look for a row of screws labeled R, C, W, W2, G, Y/Y2, like so.
As you can see in the photo above, this furnace does not have a C-wire hooked up. To make this HVAC system compatible with a smart thermostat, it would need either:
- a new bunch of wires run between the furnace and the thermostat, or
- the Venstar Add-a-Wire, or
- a smart thermostat like the ecobee3 which comes with a Power Extender Kit for systems without a C-wire
If the thought of poking around in your furnace or thermostat makes you uneasy, consider hiring a professional for the installation of your smart thermostat.
There are no standards for wire color! Any wire can be used for any purpose. A previous owner or handyman may have been “creative”, so what you find behind your thermostat might vary from what you read about online or here. These photos are from my own home, but every home is a unique snowflake.
Here are common uses for wire colors:
- Blue or Black – C – Common wire, may be unused by your existing thermostat. Enables continuous power flow from the Red wire.
- Red – R – 24VAC power from the furnace’s transformer
- Red – Rc – 24VAC (dedicated to heat call)
- Red – Rc – 24VAC (dedicated to cooling call)
- Green – G – Fan
- White – W – Heat
- Yellow – Y – Air conditioner
Do I Really Need a C Wire?
Yes, you should install a C-wire if you are upgrading your thermostat.
Nearly all modern thermostats, smart or not, require a power source, and that doesn’t look likely to change any time soon.
Nest Owners Not Exempt
The popular Nest thermostat claims to work without a C-wire, there are some caveats. Without a C-wire, the Nest gets its power from your heating or cooling system… assuming it’s running. When it’s not running, the Nest still needs to get power. The Nest will “pulse” the heat wire, turning on the furnace to pull a bit of power to keep itself going.
In some systems, this is unnoticeable, but in others, the furnace responds as if being told to turn on and then immediately off.
Nest’s manufacturer updated its literature to warn that the Nest may be incompatible with some single cycle, no-C-wire systems, but the reality remains that outdated wiring is going to become more and more of a pain in the butt to deal with.
Most states have outlawed the old fashioned “mercury blob” thermostat. Even the most no-frills thermostats on the market today still need a power source. Adding a C-wire, either through new wiring or an adapter, ensures compatibility with new thermostat technology.
Don’t do the “fan wire” trick
It is true that you can re-purpose the fan wire as a power wire, but then you (and future homeowners who live in your home) won’t be able to manually turn the fan on. If you’re going to spend a couple hundred bucks on a thermostat, spend a little more for an adapter or a technician to do the job right.
What to do if you lack a C-wire
- Hire a pro to run new wires between your furnace and your thermostat (or do it yourself)
- Grab a Venstar Add-a-Wire, which adds a 5th wire to your 4-wire setup
- Get a smart thermostat that’s designed to work with systems lacking a C-wire, like the ecobee3 (it has an adapter for C-wireless systems) or the Emerson Sensi (in most systems, it doesn’t need a C-wire at all)
- Install new thermostat but not sure if I have wire C – user post and answers on doityourself.com forum
- What is a C-wire? (by Sensi) – great explanation by the creators of one of our favorite thermostats