Wiring Details:
The thermostat uses 1 wire to control each of your HVAC system’s primary functions, such as heating, cooling, fan, etc. See the diagram below for what each wire controls on your system:
Y – Compressor Stage 1 (Cooling)
Y2 – Compressor Stage 2 (Cooling)
G – Fan
C – Common
L/A – A – Input for heat pump fault
W -O/B – Heat Stage 1 (Heating) / Reversing valve for Heat Pump systems
W2 -Aux/E – Heat Stage 2 (Heating) / Backup Heat
R – 24vac (Heating transformer)
Rc – 24vac (Cooling transformer)
At its most basic, your thermostat connects terminals via internal relays to send power to these different wires to control your HVAC system. For instance, for a gas furnace to turn on the heat, the thermostat internally connects the R terminal (power) to the W terminal (heat stage 1) to turn on the furnace.