Heat Pump: Reversing the Cycle and Auxiliary Heat
Reversing the cycle
A heat pump adds a reversing valve, so the outdoor coil becomes the evaporator in heating and the indoor coil becomes the condenser. Know which coil is which in each mode, that the suction line is always the line returning to the compressor (measured at the true suction port on the reversing valve), and that the indoor coil rejects heat in the heating mode.
Know the accessories: auxiliary heat (electric strips staged with the compressor), emergency heat (strips only, compressor locked out), balance point (where heat-pump capacity equals building load), and defrost (initiated on time/temperature or demand, terminated by coil temperature or time, with strips energized during defrost to temper supply air).
Traps: checking charge in heating mode without the manufacturer's chart (weigh in, or verify subcooling in cooling mode), condemning a reversing valve when the real fault is the solenoid coil or a valve that needs a pressure differential to shift, confusing auxiliary heat with emergency heat, and forgetting that low indoor airflow in heating raises head pressure and temperature rise.
Worked example: at 30°F outdoor the heat pump delivers 24,000 Btu/hr while the house load is 30,000 Btu/hr. The 6,000 Btu/hr gap must be made up by resistance heat: 6,000 ÷ 3,412 ≈ 1.8 kW, so at least a 2 kW strip bank must stage on, with lockout controls deciding when.