C-wire
A C-wire (common wire) is a low-voltage wire that gives a thermostat continuous power from your heating and cooling system.
What it means for you
Most smart thermostats need continuous power, so check whether your wall has a C-wire before you buy. If it does not, some makers sell an adapter kit, and our thermostat guide covers the checks.
Guides on this topic
- Best Smart Thermostats
C-wire needs, HVAC compatibility, Matter and platform support, and rebate tips.
- ecobee Thermostat Setup and Wi-Fi Fixes
ecobee Wi-Fi setup and drops, blank or rebooting screens, the Power Extender Kit and SmartSensor pairing, from ecobee support.
- Google Nest Thermostat Offline and Wi-Fi Fixes
Nest thermostat offline in the Google Home app: battery, restarts, signal strength, hotspot test, network reset, router settings and the W5 code, from Google's Nest help.
Related terms
← All glossary terms · Search all guides for “C-wire”
Definitions are short and general. For model-specific details, check your device maker’s own documentation.