Verify each thermostat terminal connection before powering the HVAC system. The low-voltage control cable normally contains conductors linked to terminals labeled R, C, Y, G, and O or B. The R terminal carries 24-volt supply from the transformer, while C provides the common return path that completes the control circuit between the air handler and the outdoor unit.
Cooling and reversing valve control lines require careful placement. The Y conductor sends a signal from the thermostat to the compressor contactor, activating the outdoor compressor and fan assembly. The O or B terminal manages the reversing valve that changes refrigerant flow direction between cooling and heating operation.
Indoor air movement depends on the G conductor connected to the blower relay inside the air handler cabinet. When the thermostat calls for airflow, this lead energizes the relay coil, closing contacts that supply line voltage to the blower motor. Secure connections at the control board prevent intermittent operation or fan cycling issues.
High-voltage supply inside the outdoor condenser cabinet usually involves a two-pole disconnect feeding the contactor. When the thermostat sends a cooling signal through the Y lead, the contactor coil receives 24 volts and pulls the contacts closed, delivering line voltage to the compressor and condenser fan motor.
Rheem Heat Pump Wiring Diagram for Thermostat Compressor and Control Board Connections
Confirm every thermostat terminal before applying power to the climate system. The low-voltage cable usually includes conductors linked to R, C, Y, G, and O or B posts. The R lead delivers 24-volt supply from the transformer located inside the air handler cabinet, while the C conductor provides the common return path that completes the control loop between indoor and outdoor sections.
The Y conductor carries the cooling call from the wall controller to the outdoor compressor contactor. When this line receives 24 volts, the contactor coil energizes and pulls the internal contacts closed. This action sends line voltage from the disconnect block directly to the compressor and condenser fan motor. A loose connection at this lead often results in outdoor unit silence during cooling demand.
The G terminal controls the indoor blower relay. Once the wall controller sends a signal through this conductor, the relay coil activates and closes the contacts supplying 120 or 240 volts to the fan motor inside the air handler. Verify continuity between the thermostat terminal and the board input pin; resistance above a few ohms may indicate a damaged conductor.
Reversing Valve and Mode Switching
The O or B conductor controls the reversing valve solenoid mounted on the outdoor refrigerant assembly. Many units energize the valve during cooling mode through the O terminal, while others use the B terminal during heating mode. Always check the equipment label near the control board because incorrect polarity keeps the system stuck in one operating mode.
Inside the indoor cabinet, the control board distributes the 24-volt signals to relays and safety switches. Pressure switches, defrost control modules, and limit sensors connect to this board through spade connectors or multi-pin plugs. When any safety device opens, the board interrupts the Y signal, stopping the compressor contactor.
Defrost control modules also connect through the same low-voltage network. During winter operation, sensors monitor coil temperature and elapsed compressor run time. Once frost accumulation reaches a programmed threshold, the board activates the reversing valve and outdoor fan relay for a short defrost cycle.
Compressor Contactor and High Voltage Path
The compressor receives line voltage through a two-pole contactor located in the outdoor cabinet. Incoming power from the disconnect box attaches to the L1 and L2 terminals, while outgoing leads connect to the compressor and fan motor. The contactor coil connects to the Y signal and the C return line, creating a 24-volt activation loop.
Secure grounding between the outdoor chassis, indoor cabinet, and supply panel. A loose ground strap may allow voltage fluctuations across the control board and relay coils. Tighten each ground screw and verify continuity below one ohm between metal panels and the equipment grounding conductor.
Thermostat Terminal Wiring for Rheem Heat Pump R C Y O B and G Connections
Connect the R terminal from the thermostat directly to the 24-volt supply lead from the indoor air handler transformer. This conductor carries constant low-voltage power used by the control system. A typical cable uses a red conductor for this terminal. Measure between the R terminal and C using a multimeter; the reading should remain near 24 volts AC during standby.
Primary Low Voltage Power Terminals
The common terminal marked C completes the low-voltage control loop between indoor and outdoor sections. Without this return path, the control board cannot energize relays or the compressor contactor coil. Many modern thermostats with digital displays require both R and C conductors.
- R – 24V supply from transformer
- C – common return line for control voltage
- Typical cable colors: red for R, blue or brown for C
- Voltage measured between R and C should remain near 24V AC
The Y terminal activates the compressor section outside the building. When the thermostat calls for cooling, it sends 24 volts from R through the Y conductor toward the outdoor contactor coil. The contactor then closes, delivering line voltage to the compressor motor and condenser fan.
The G terminal controls the indoor blower relay inside the air handler cabinet. Once the thermostat closes the R-to-G connection, the relay coil energizes and powers the fan motor. Verify that the blower starts immediately after the thermostat calls for airflow.
Mode Control Through Reversing Valve Leads
Terminals marked O or B operate the reversing valve solenoid mounted on the outdoor refrigerant assembly. The thermostat energizes one of these leads depending on equipment configuration.
- O terminal – energized during cooling on many units
- B terminal – energized during heating on some systems
- Only one of these terminals is normally active
- Incorrect placement results in reversed operation
Confirm polarity by observing system behavior. If the unit produces warm air during a cooling request, the reversing valve conductor likely sits on the wrong terminal.
Secure all thermostat screw terminals tightly and avoid exposed copper beyond the clamp point. Poor contact increases resistance and can prevent relays from receiving full 24-volt control power, leading to intermittent compressor or fan activation.