Why your current thermostat is not using the common wire (C-wire)

There could be several reasons why your current thermostat is not using the common wire (C-wire) in the wiring harness to the upstairs split heat AC unit:

1. Your thermostat may not require a C-wire to function. Many older or simpler thermostats, especially non-smart models, can operate without a C-wire as they don’t need constant power3.
2. The C-wire may be present in the wiring bundle but not connected. Sometimes, the C-wire is left unused and tucked away in the wall4.
3. Your thermostat might be battery-powered. Some thermostats are designed to run on batteries alone, eliminating the need for a C-wire connection5.
4. The thermostat could be using alternative power methods. Some smart thermostats can operate without a C-wire by using “power stealing” techniques, drawing small amounts of power from other wires4.
5. Your HVAC system might not have a C-wire terminal. In some older systems, especially those with only heating, a C-wire may not be available2.

To determine the exact reason, you would need to:

1. Check your thermostat’s model and specifications.
2. Inspect the wiring behind the thermostat to see if there’s an unused C-wire.
3. Verify the wiring configuration at your HVAC control board.