C-Wire 101: What to Know

What are Common Summer Thermostat Problems?A c-wire is an important detail to be aware of if you’re upgrading to a new, smart thermostat. Unlike the old-school, battery-operated thermostats they’re rapidly replacing, these units incorporate intelligent technology—such as Wi-Fi connection options, a multifunctional color touchscreen display and integration into home automation systems—which requires continuous 24-volt electrical power. Short for “common wire,” a c-wire is the extra wire necessary to complete the circuit that provides that power.

Many newer homes are already pre-wired with a c-wire connected to the HVAC system. In that case, upgrading to a smart thermostat is straightforward. However, if your house (like many) isn’t, installation of the new thermostat will require a minor extra step to get it installed. Fortunately, there’s an easy way to determine right now whether you’re already “c-wired” or not.

    • Remove the existing thermostat from its wall mounting bracket. It’s the same procedure you’ve probably done before to replace batteries.
    • Check out the wiring that extends from the wall and connects to the screw terminals in the thermostat bracket.  Count how many different wires you see, including any that may be unused—hanging loose or folded up.
    • If you count five wires, one is likely the c-wire so you won’t need to have it installed to power the new smart thermostat.
    • If you count only four wires, this means you’ll probably need an extra wire installed to function as a c-wire.
    • Note: you can’t rely on wire color to identify the wire. There’s no uniform standard color for a c-wire.

Suppose you only see four wires.  Now what? Installation of the required c-wire is a straightforward and familiar procedure for an experienced HVAC service technician. In most cases, the new c-wire will connect to the control board at your furnace or air conditioner to complete the circuit that provides continuous 24-volts for your new thermostat.

Contact Powers Heating & Air for more information about upgrading to a new smart thermostat and c-wire installation that may be required.

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