The common letter conventions for 24 V control wiring between a thermostat and equipment.
Common conventions: R 24 V hot from the transformer, C transformer common, Y cooling (compressor contactor), G indoor fan, W heat, and O or B the heat pump reversing valve. A cooling call with the fan on automatic connects R to Y and G. Manufacturers vary, so confirm against the wiring diagram on the equipment.
Exam work areas
D · Installing systems and controls F · Maintenance, service and troubleshooting