What it is
Your thermostat talks to the heating and cooling system over low-voltage (24 V) wires. The C-wire, short for common, completes the circuit so the thermostat gets constant power for its screen and Wi-Fi. Without it, some smart thermostats “steal” power and can cause the system to short-cycle or the thermostat to drop offline.
How to check
- Turn off power to the heating and cooling system at its switch or breaker.
- Pull the current thermostat off its base plate.
- Look for a wire in a terminal labeled C. Many smart thermostat makers’ apps walk you through photographing the wiring to check compatibility.
- If no wire is on C, look for an unused wire tucked behind the plate. It may be connected at the furnace end, or not.
If the wires are thick, or the thermostat says 120 V or 240 V, it’s a line-voltage thermostat for baseboard heat. Stop: that’s a different product, and a job for an electrician if you’re unsure.
If you don't have one
- Use the maker’s power adapter. ecobee includes one with many models; Amazon and Google sell C-wire adapters for theirs. They install at the furnace.
- Use a spare wire. If there’s an unused wire in the bundle, it can become the C-wire at both ends.
- Pick a thermostat that doesn’t need one on your system, such as some Sensi models. Check the maker’s compatibility tool.
- Run a new wire. An HVAC technician can do it if none of the above works.