If you’ve done any research at all, you know that not all HVAC systems are installed with a thermostat c-wire, and that causes much consternation when selecting and installing a thermostat. This post explains:
- The purpose of the c-wire
- Identifying the c-wire in your thermostat
- Options if you don’t have a c-wire
- Thermostat buying tips related to the c-wire
What the C-wire Does
The c in c-wire stands for common. The purpose of the thermostat c-wire is to provide continuous 24-volt AC power, known as 24VAC, to the thermostat. And the purpose of having continuous power is to get a constant, accurate thermostat reading.
The wire originates in the furnace or air handler and runs to the thermostat in the bundle or wires, but only IF your system was installed with a common wire.
Identifying the Common Wire
Does your thermostat have a c-wire? Let’s find out. Normally, the blue wire in the bundle is used for a common wire. However, all the colored wires are the same, so it doesn’t matter what color wire is used for each function – heating, AC, fan, etc., if it is attached where it should be attached on each end.
The first step is to remove the thermostat from its mounting base to expose the wires and the terminal that many or all of them are attached to. Find the C terminal. Is there a wire connected to it? That’s the common wire, and in most cases it will be blue. If there isn’t a wire connected to the C terminal, gently pull the bundle of wiring from the wall to see if there is a C wire in the bundle that simply isn’t connected. If there is, see option 2 below for connecting a C wire to the air handler/furnace and thermostat.
You’re good to go, and you can now install any 24VAC thermostat you want including nest, ecobee, lyric or other WiFi thermostat. Our Thermostat Buying Guide offers useful tips for selecting the right thermostat and determining compatibility with your system.
Options If You Don’t Have a C Wire
No C wire? No worries, as they say. You’ve got five options if you do not have a c-wire.
Buy a thermostat that doesn’t need one. The manufacturers of the nest thermostat and Lux Geo thermostat, as two examples, claim their products run properly with or without a c-wire. Technically, they are correct. While the thermostat won’t have continuous power, it can borrow power from the HVAC system when it is running. When the system is off, the thermostat will signal it to power up very briefly at regular intervals. This gives the thermostat the power it needs to check your home’s temperature and know if a heating or air conditioning cycle is needed.
In short, you can use some thermostats without a c-wire, but they won’t be as accurate, and you might experience slight temperature swings that wouldn’t occur if the thermostat was constantly powered and therefore constantly knew what the temperature was.
Option 2: Use an unused wire for the c-wire
Is there an unused wire in the bundle behind your thermostat? If so, the end of that wire should be unconnected at the other end too. We recommend hiring a pro to make the connection because it involves electrical power and the risk of shock If you go it alone, turn off the circuit used by the HVAC system. Adjust the thermostat to call for heating or cooling to make sure the system won’t come on. Then, open the cabinet of the furnace or air handler, locate the wiring bundle and find the unconnected wire. Attach it to the C terminal there and at the thermostat.
Option 3: Use the G wire as a C wire
The G wire controls the fan independently. You’ll lose fan-only mode if you do this, and the fan will only run when a heating or air conditioning cycle is running. Again, a pro can do this if you’re not confident about the DIY approach.
Option 4: Hire an HVAC technician to install a common wire, and never have this hassle again
It’s a pretty simple job. Hiring a pro for any of the suggested tasks will cost $100-$300, typically. You can use our estimate tool to get accurate local thermostat installation cost.
Option 5: Use a splitter/jumper module as a work-around to installing a c-wire
Here are popular brands:
- Honeywell Wire Saver Module
- Sensi Common Wire Kit
- ecobee Power Extender Kit
- Venstar Add-a-Wire
Each thermostat that requires one of these kits has slightly different directions, like these from Ecobee, and they should be easy to follow. The manufacturer websites also have videos, so you can watch it being done. The purpose of these splitters is to borrow power from one of the other wires in the thermostat wiring bundle, usually the Rh wire used for the furnace or air handler.
Here is video of how to install Venstar’s Add-A-Wire:
Your Best Options
The bottom line is that you have options, and most are pretty low on hassle and cost. Most homeowners get by with option 1, and if they don’t like the performance of their thermostat without a C wire, they first try one of the kits listed in option 5. Your local HVAC pro will be happy to help, too.
Has this information been useful? Perhaps your friends thinking about a smart thermostat would benefit from it too if you pass it along on social media!
4 thoughts on “C-Wire Issue: What If I Don’t Have a C-Wire”
C means COMMON is like the C in BCE vs. BC. It stood for Christ in BC and now stands for Common in BCE. I’ve come across old Honeywell timer thermostat manuals from the 1950s or 1960s, C stands for Clock. The terminal provided continuous 24V power, along with the R terminal, to drive the thermostat’s electric clock motor, which drove the heat/cool schedule disk.
To my thinking, CLOCK has always made a lot more sense than Common.
Right on, John! You the man!
Loved ” It stood for Christ in “BC””. 🙂
Great sense of humor.
John, this actually helped a lot. I was trying to fix a friend’s smart thermostat, and couldn’t understand why ‘common’ wouldn’t be the one that is used for the controls. I only had a schematic on an Intertherm furnace, and colors that didn’t seem to match anyone else online. Thanks! Got the furnace working and the fan control also!
My aint is paying $100/month for internet just so she can use a thermostat on her Trane that will operate via her phone so she doesnt have to walk across the house to change temperature. I told her she should replace it with a bluetooth thermostat and save the $100 internet bill. I believe she needs one with the C-wire requirement though. She lives in a well covered cellular tower area so she has full signal strength on her cell phone. She still pays for regular home phone service too for some reason.