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Thermostat Wiring Diagrams

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A typical furnace + AC uses five wires: R (24V power), C (common), G (fan), Y (cooling) and W (heat), each landing on the matching letter at the furnace board, with Y and C continuing to the outdoor unit. A heat pump adds O/B (reversing valve) and AUX/W2 (electric backup heat). Match terminal letters, not wire colors.

Prints all three diagrams and the terminal key.

Turn off power to the equipment before wiring. These are typical layouts; always follow the wiring diagram on the equipment and the thermostat manufacturer's instructions.
ThermostatRCGYWFurnace boardRCGYWAC condenserContactor coil(Y)(C)
1. Gas furnace + central AC (conventional, 5 wires)
ThermostatRCGYO/BAUXAir handlerRCGYO/BWHeat pumpRCYO/B
2. Heat pump + air handler with electric aux heat (AUX → heat strips W)

Some heat pumps also run a wire from the outdoor defrost board to the air handler's heat strips (often labeled W or X) to temper air during defrost. Check the unit's diagram.

ThermostatRWBoiler / valveTT (R)TT (W)
3. Heat only, 2 wires (boiler, zone valve or older furnace)

Terminal key

TerminalWhat it does
R (Rh, Rc)24 VAC hot from the transformer. Rh/Rc split heating and cooling power on systems with two transformers; a jumper joins them on single-transformer systems.
C24 VAC common, the return side of the transformer. Powers Wi-Fi and smart thermostats continuously.
GIndoor blower fan.
Y (Y1)Cooling: energizes the outdoor contactor (and the compressor on a heat pump in both modes). Y2 = second stage.
W (W1)Heat: furnace or boiler call. W2 = second stage heat.
O/BHeat pump reversing valve. O energizes in cooling (most brands); B energizes in heating (Rheem/Ruud and some others). Set the thermostat to match.
AUX / W2 / EHeat pump auxiliary electric heat. E is emergency heat (aux without the compressor); many installs land AUX and E on the same air handler terminal.

Wire colors shown are common convention only. Installers don't always follow them.

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