What is a C wire?

The C wire is another name for the Common wire. There is no universal color used to designate this type of wire. It is simply a wire that originates from your heating/cooling system and needs to be connected to the C terminal on your Lyric, Wi-Fi or connected thermostat. The purpose of the C wire is to provide constant 24-volt power to the thermostat.

Not all thermostats need a C wire, although use of Apple HomeKit with your thermostat and most Wi-Fi enabled thermostats do require one. The Honeywell Lyric Round Wi-Fi Thermostat is the one exception to the Wi-Fi, smart, connected thermostat rule, as it does not require a C wire. It can still function by drawing power from your heating and cooling systems to power its battery. This method is also referred to as Phantom Power or Power Stealing. Honeywell also offers a wide variety of non-connected thermostats that do not require a C wire to operate.

If a C wire is required for your thermostat install, please read: What options do I have if I don't have the "C" or common wire?

For more information on the C wire topic, please watch this instructional video:

Last updated: July 27, 2018
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