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.
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.
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.
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.
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- 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.
