Heat Pump Electrical Controls and Defrost
This domain tests low-voltage control circuits and the defrost sequence. You need to know thermostat terminals, defrost board inputs and outputs, sensors, reversing-valve logic, auxiliary heat staging, and safe troubleshooting of line- and low-voltage components.
Control basics
- Typical terminals: R (24 V hot), C (common), Y (compressor), G (fan), O/B (reversing valve), W (auxiliary/emergency heat). Heat pumps often use O for cooling changeover and B for heating changeover; the thermostat must be configured correctly.
- The defrost board monitors outdoor coil temperature and compressor run time. Time-temperature boards initiate defrost on accumulated run time and terminate on coil temperature or a time limit. Demand-defrost boards use sensors and logic to initiate only when frost is present.
- During defrost, the reversing valve shifts to cooling, the outdoor fan stops, and auxiliary heat is energized to temper supply air. The board may terminate on coil sensor temperature (often around 50–60°F) or a maximum time.
- Crankcase heaters, compressor time delays, high/low-pressure switches, and float switches protect the system.
Worked example
A heat pump heats poorly and the outdoor coil is frosted. The defrost board has 24 V at the reversing-valve output during defrost, but the valve does not shift. Check the valve solenoid coil resistance and the common wire at the valve. If the coil is open, the board is commanding correctly and the solenoid is the fault. If no voltage appears, check the board's defrost sensor and timer before condemning the valve.
Traps
Do not assume O/B is universal; miswiring causes heating in cooling mode. Do not jumper safety switches. Do not diagnose a defrost sensor as a thermostat problem, and remember the outdoor fan should stop during defrost.