HVAC thermostat wiring, the common wire is for what exactly

The common wire, also known as the C-wire, in furnace and HVAC thermostat wiring serves a specific purpose:

1. It provides a constant 24-volt AC power supply to the thermostat15.
2. The C-wire completes the circuit in the cooling and heating system, allowing the thermostat to receive continuous power and function effectively2.
3. It acts as a return path for electrical current, ensuring the thermostat has a stable power source at all times25.

This constant power supply is particularly important for modern smart thermostats and those with advanced features like Wi-Fi connectivity, touch screens, and programmable settings2. The C-wire eliminates the need for batteries or power-stealing techniques, which can lead to more stable operation and prevent potential system malfunctions24.