Learn about the common wire and why a Nest thermostat may need one

Nest thermostats are engineered to use very little power: less than 1 kWh/month. For comparison a typical 40W light bulb that's used 4 hours per day consumes about 5 kWh/month. So even though Nest thermostats have Wi-Fi activity sensors, color displays, and more, they typically don't need a C wire to stay charged up.

The Nest app gives you a wiring diagram during the setup to connect your Nest thermostat to your system.

If your current thermostat has a wire in the C terminal: the wiring diagram that you'll get from the Nest app will tell you to connect the C wire to your Nest thermostat. If your current thermostat doesn't have a wire in the C terminal: the Nest app won't tell you to connect one. Do not connect a C wire to your Nest thermostat unless the Nest app tells you to.

 

In some cases, you may experience odd heating or cooling behaviors after installing your Nest thermostat. Connecting a C wire to your Nest thermostat may help resolve the following issues:

• Your system is making strange noises: chattering, stuttering, clicking or thumping - this can be caused by your system turning on and off rapidly
• Heating or cooling is always on, and won’t turn off
• Heating or cooling repeatedly turns on and off in a short period
• You see a Delayed or Starts in message on your Nest thermostat’s screen (for example: “Heating is delayed for 2:30 mins”)
• The system fan is always running or won’t turn on
• The system fan turns on and off repeatedly in a short period

The best solution for the above issues is to contact a Nest Pro in your area. They can diagnose and help fix any problems with your system and thermostat installation. If needed, they can also connect a common (C) wire or a C wire accessory, such as a Fast-Stat Common Maker, to your Nest thermostat.

See the following article for more information:

Troubleshooting strange heating and cooling behaviors