A C-wire is required for many smart thermostats because they need a constant flow of electricity from the furnace in order to keep the smart components running.

Smart thermostats usually have Wi-Fi in them.

This is so they can communicate to the internet, which allows you to monitor or change the temperature when you are away.

In addition, smart thermostats also have a lit color display, which uses power; therefore, batteries will not provide enough power for a long enough period of time to be useful, thus the need for the C-wire power from the furnace or boiler.

Not all smart thermostats require a C-wire. Some newer models include circuitry that captures the intermittent power from the red wire to charge an internal battery; this means they can run entirely without a common wire, which makes installation a lot easier.

For instance, the Nest Thermostat and the Nest E Thermostat can usually run without a common wire — see below for more information.

Other thermostats, like the Ecobee, come with an adapter that can be used if you already have four wires, but no common wire — Ecobee thermostats are also discussed below.

Photo of an Amazon Smart Thermostat with C-Wire Power Adapter
  • Amazon Smart Thermostat is an easy way to switch from a traditional thermostat and help reduce energy usage.
  • Amazon Smart Thermostat is ENERGY STAR® certified.
  • Compatible with most 24V HVAC systems
  • Enable Thermostat Hunches in the Alexa app

Is RC and C wire the same?

Depending on the model and version of your smart thermostat, it may have variations in the wires.

Many devices have RH wires for heating and RC wires for cooling. Or, they have a single R H/C wire. Then again, you might notice that the c wire in thermostat is black in color in few devices. 

A good rule of thumb is to look for the “C” mark to identify the correct wire. In case the electrician rewired the device to add a common wire, they may have used an entirely different colored wire. But the functionality will remain the same. 

Do keep in mind that if your smart thermostat is battery-operated, it may not need the C wire. If you don’t see one, don’t worry about it. And, to answer your question, the RC and C wires are entirely distinct.

Check out this easy-to-understand table for identifying the wires in your smart thermostat:

  • Black or Blue wire – Common “C” wire
  • Red – “R” or Hot wires that connect to the furnace transformer via 24 VAC power
  • Red – “RC” connects to the cooling system via 24 VAC power
  • Red – “RH” connects to the heating system via 24 VAC power
  • Yellow – “Y” connects to the air conditioning
  • Green – “G” connects to the fan
  • White – “W” connects to the heat
Thermostat wires diagram with C wire

Where does C Wire Go on Thermostat?

The C wire or blue wire in thermostat connects with the system’s power supply. You’ll remove the face-plate of your smart thermostat by simply dislodging it from the wall mounting plate.

Check the circuit behind it, and you will see the thermostat C wire, as explained in the section above. 

What if there is no C wire? Here are your options:

  • If you intend to install a battery-powered thermostat, you don’t need a C wire since it likely does not have a touch screen. 
  • If you wish to install a Wi-Fi-controlled smart thermostat, you’ll need to organize a C wire for powering the device. 
  • Even if you don’t see the thermostat C wire, chances are that there is an extra one left disconnected behind the device. Remove the mounting plate and examine the gap behind it. Identify the wire and make a note of its color. Next, examine your HVAC control panel and look for the wire attached to the “C” terminal. If it has a similar colored wire, that’s the one you need.
  • Also, check the furnace control panel. If it has a C wire, your thermostat may have one also.
  • If you don’t have a C wire, your technician will likely install one separately. You can try a DIY project, and the process is simple, but remember that the device uses low voltage power. This is why you must be extremely cautious. 
  • The SunTouch SunStat Command has an intuitive schedule that makes programming simple.
  • Easy and reliable way to control your floor heating system
  • Intuitive 7-day schedule makes it simple to program floor heating
  • Dual Voltage 120/240V with built-in GFCI for safety

Do I Need to Connect the Blue Wire in Thermostat?

Yes, to have a fully operating temperature regulating system, you’ll need to install a c wire in thermostat. There are several methods to execute this process. But, before you start, follow the safety steps carefully. Remember to power off the furnace and thermostat at the circuit breaker.

Check if the furnace and thermostat are working by turning them on and off. If they don’t work, you’ll know that it is safe to proceed with your DIY task.

When looking for the blue wire in thermostat, remember that the color-coding explained above is merely a guideline. The technicians originally working on your electrical systems may have chosen to get creative with the wires.

Sorting and connecting them with the proper terminals may take longer, but it can be done. Ready to get started?

Method 1 – Creating a New C Wire Connection with Your Furnace

  1. Remove the face-plate from the furnace control panel and check the wires and terminals.
  2. Remove the face-plate and mounting plate from the thermostat and set them aside to expose the gap in the wall.
  3. Select an 18 AWG wire and attach it to the C terminal in the furnace control panel. 
  4. Thread it through the wall toward the thermostat location. It may take you longer, depending on the distance of the furnace from the thermostat.
  5. Connect the C wire securely to the mounting panel terminal. 
  6. Replace the face-plate. Your thermostat C wire is installed and ready for use.

Method-2 – Using an Existing Spare Wire

  1. After removing the face-plate, examine the wires in the thermostat. Chances are that there is an extra unused wire in the space. For instance, depending on the climate conditions where you live, you may not use the G terminal for a fan. This one can serve as your thermostat C wire.
  2. Attach it to the C terminal, and you’re good to go.

Method 3 – Replacing the Entire Wiring with a New 18/5 Wire

  1. Replacing the old thermostat C wire is always a smart option, but that will work only if your furnace and HVAC control panels have a C terminal. If they don’t, it is advisable to call in a professional technician to replace the wiring for you.
  2. Remove the control panel of your HVAC systems to expose the wires. 
  3. If you see a blue wire in thermostat and C terminal, begin by disconnecting the existing wires. Remove the wall mounting plate to expose the gap in the wall.
  4. Attach the end of the new wire securely to the end of the old wire. Getting an 18/5 wire will give you a bundle of five differently-colored wires to connect to the terminals.
  5. Remove the control panel and mounting wall panel of the thermostat. Disconnect the wires from the terminals, leaving them loose.
  6. Tug gently at the ends of the old wire to draw out the new wire until it appears at this end. 
  7. Connect the wires to the proper terminals using the color-coding guide explained in the preceding sections. 
  8. Remember to use the blue wire as the thermostat C wire. 
  9. Replace the face-plates on the HVAC systems and thermostat. 
  10. Your new smart thermostat is now ready for use. 

How Do I Know If My C Wire is Working?

The best way to check if your thermostat C wire is working efficiently is to use a multimeter tool. Set the reading to Volts and AC and test the voltage coming down to the thermostat.

Place the red pin on the R terminal and then place the black on the C terminal. If the multimeter registers anywhere from 25V to 27V, you’ll know that your smart thermostat is all set and working in perfect condition.


Having a workable smart thermostat presents many advantages for the average property owner. You can conserve energy costs and control the temperature in your home or office from a remote location.

When installing the device, you must check the wiring and look for a blue wire. This thermostat C wire serves as the power regulator and ensures that the thermostat will function just as it is designed to work.

Adrian Dance

You might call me a techie or a technology enthusiast. I’m one of those people whose largely self-taught in the worlds of computing, technology and engineering. I have a DIY approach to the use of smart home automation and think that with the right advice, anyone can improve their lives through greater use of technology.