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

In most cases, Google Nest thermostats can charge their built-in battery using your system’s heating and cooling wires. But in a small number of situations you may need to connect a common or C wire to deliver enough consistent power to your thermostat.

A common wire is simply a wire that's dedicated to delivering constant power to your thermostat. Unlike the other wires connected to your thermostat it doesn't control heating or cooling functions.

Why Nest thermostats don't typically need a C wire

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 compatibility checker will tell you to connect a C wire

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.
 Nest thermostat wiring diagram

 

Why your Nest thermostat may need a C wire

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 local pro. 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

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