What is a C wire or common wire? 

If you’re considering a smart thermostat, a C wire (common wire) may be required.  We’re here to guide you through the process to determine if a thermostat C wire is required for smart thermostat installation.

Most smart thermostats today that are Wi-Fi enabled or have a color touch screen may require constant power through a thermostat C wire. This ensures your thermostat has a continuous flow of power to function properly.  

There may be a few smart thermostat models that don’t require a C wire and can operate with batteries. You should always ensure proper wiring as needed for your HVAC system and corresponding thermostat (with or without a C wire). Check your HVAC system compatibility to see if you need a C wire for your smart thermostat.  You can also reference our complete guide and videos to the C wire.   

How can I tell if my home has a C wire?  

As a rule of thumb, newer homes (built in the last 40 years) generally have the proper wiring for installing any smart thermostat, including a C wire. To confirm, take a look at your current thermostat's wiring by removing the faceplate. Most faceplates can be easily removed by simply pulling it straight away from the mounting plate. You’ll see several labeled wire terminals. Use our handy wire picker tool to determine if you have a C wire and which Sensi smart thermostats are compatible with your system.  

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What color is the C wire?   

The C wire's color is not standard across all types of thermostats. But most commonly, systems have blue or black C wires. Traditional HVAC thermostat wiring in our homes uses four or more wires.

Thermostat wiring color

Wire coloring is not standard across all types of thermostats, but these are the most common colors:

• C wire (blue or black wiring, common for 24-volt power to the thermostat)
• R wire (red wire for power)
  • R - 24 VAC power from the transformer in your gas furnace
  • Rc - 24 VAC provides power to cooling
  • Rh - 24 VAC provides power to heating
• G wire (green wire for fan)
• Y wire (yellow wire for air conditioning)
• W wire (white wire for heat)

What if I don’t have a common wire?   

Conventional heating systems and cooling systems do not require a common wire, but heat only, cool only (air conditioning), and heat pump systems do require a common wire. Check your compatibility in the chart below. 

Sensi Compatibility Chart

System Type	Sensi Lite smart thermostat	Sensi smart thermostat	Sensi Touch smart thermostat	Sensi Touch 2 smart thermostat
Conventional Heating and Cooling          - Gas/Oil/Propane Furnace          - Air Conditioner          - Electric Furnace          - Boiler/Radiant	Yes1	Yes2	Yes3	Yes3
Heat Only          - Gas/Oil/Propane Furnace          - Electric Furnace          - Boiler/Radiant	Yes3	Yes3	Yes3	Yes3
Cool Only          - Air Conditioner	Yes3	Yes3	Yes3	Yes3
Heat Pump/Geothermal	Yes1,3	Yes3	Yes3	Yes3
Whole Home Humidifiers / Dehumidifiers	No	Yes	Yes	Yes
Communicating/Proprietary System	No	No	No	No
Line Voltage	No	No	No	No
Millivolt	No	No	No	No
RV/Camper 12VDC Applications	No	No	No	No

¹ Compatible with multi-stage systems up to 1H/2C or 2H/1C
² Common wire required for HomeKit compatibility
³ Common wire required 

If you don’t have a C wire connection and have no desire to add one, the Sensi smart thermostat and the Sensi Lite smart thermostat are a nice option that are powered by batteries and do not require a C wire*.  If you would rather have the Sensi Touch smart thermostat or Sensi Touch 2 smart thermostat, a C wire is required. 

Options for adding a C wire

Option 1: Check for an existing wire

There may be one wire already available that was not used by your old thermostat. If there is an extra, unused wire, check the color code in the wiring diagram of your HVAC unit to ensure proper setup.

Option 2: Use G wire or fan wire as the C wire

In some cases, you can use a G wire or fan wire as your C wire. The Sensi app will walk you through the setup with the wires you have to see what can be converted to a C wire.

Option 3: Install a C wire conversion kit

This is an easy option that allows you to use your four wires without running any additional thermostat wiring to your HVAC unit. You can purchase a Sensi conversion kit from Amazon or The Home Depot. 

For additional information and detailed instructions on adding a C wire, view our video tutorials 

Sensi thermostats are continually voted as one of the best Wi-Fi thermostats available. 

• SAVE ABOUT 23% ON HVAC ENERGY: The ENERGY STAR-certified Sensi programmable thermostat can help you save energy with features like flexible scheduling, geofencing, remote access and usage reports. 
• EASY DIY INSTALLATION: Homeowners can use the built-in level, easy-click terminals and step-by-step app instructions for a quick installation of their new thermostat. Works with the HVAC equipment found in most homes. 
• SMART MAINTENANCE: Sensi can help monitor the performance and efficiency of your HVAC system by delivering valuable usage reports, alerts about your equipment, and maintenance reminders like filter replacement. 
• SMART HOME COMPATIBLE: Sensi works directly with the most popular smart home systems on the market — including Google Assistant, Apple HomeKit, Samsung SmartThings, and Amazon Alexa. 

With over 100 years of HVAC experience, Sensi is a brand you can trust to put on your wall.

Still have questions? Contact an HVAC professional.     

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No C-wire required*

No C-wire required*

C wire required

C-wire required

*Systems that require a common wire include heat-only, cool-only, and heat pump systems