White retro-style refrigerator beside a kitchen sink cabinet
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Field manual · Air Conditioner

Split heat pump: electrical basics field manual (reversing valve coil & defrost board)

Covers: Residential single-stage split heat pumps with a time-temperature or demand defrost board and a conventional heat pump thermostat. Electrical side only; the refrigerant circuit is out of scope.

Single sourceBrands: All7 components

A heat pump is a central AC that can run backwards. The extra parts that matter for electrical diagnosis are the reversing valve solenoid coil, the defrost control board and its coil sensor, and the backup electric heat. Most 'heat pump blows cold' or 'iced outdoor coil' calls come down to thermostat configuration, the solenoid coil, the defrost sensor or board, or an outdoor fan problem.

Safety first

  • Open the outdoor disconnect and breaker and verify dead before handling boards or coils; single-pole contactors leave a leg live.
  • Discharge capacitors before touching them.
  • Live 24 V tests are low voltage, but the same box contains 240 V: keep probes clear of line terminals.
  • Reversing valve body replacement, compressor work, gauges, charge checks and leak repair are EPA Section 608 work. Diagnose and refer only.

How it works

A reversing valve in the outdoor unit swaps the roles of the indoor and outdoor coils. Its solenoid coil is switched by the thermostat's O/B wire. On most brands the coil is energized in cooling (O); a few brands, commonly reported as Rheem/Ruud, energize it in heating (B). The thermostat must be set to match or the system heats when cooling is called and the reverse.

In heating, the outdoor coil gets cold and collects frost in damp weather. The defrost board periodically puts the system into defrost: it energizes the reversing valve into cooling, stops the outdoor fan so the coil heats up, and usually energizes W2 to turn on indoor electric heat so cold air is not blown into the house.

Time-temperature boards start counting run time once a coil sensor or snap-disk thermostat says the coil is cold (often about 32 °F), and trigger defrost at the selected interval (commonly 30, 60 or 90 minutes). Defrost ends when the coil warms to a termination temperature or a maximum time passes. Termination points vary widely by manufacturer: use the OEM chart.

Demand-defrost boards compare outdoor air and coil temperatures and only defrost when the difference shows frost buildup.

Backup heat: W2/AUX brings on electric strips automatically when the heat pump can't keep up or during defrost. E (emergency heat) is a manual mode that locks out the outdoor unit and runs only the strips. Emergency heat does not melt an iced outdoor coil.

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Official manufacturer documentation

Always confirm values against the tech sheet for the exact model.

OnCall Tech is a reference tool for trained technicians and can make mistakes. Always disconnect power and confirm with the manufacturer's documentation before you work. © 2026 OnCall Tech. Original content; not a reproduction of any manufacturer document.Generic part photos (not of this brand): White retro-style refrigerator beside a kitchen sink cabinet by Pavel Danilyuk, Pexels License

Diagnostic guides for this appliance

Guide · Air Conditioner

PTAC not cooling (fan runs, air not cold)

Most PTAC no-cool calls are airflow (filter, coils), a wrong control setting or a failed compressor start component. Rule those out before suspecting the sealed system.

6 steps ~60 minmoderate
Guide · Air Conditioner

PTAC not heating (electric heat or heat pump)

PTACs heat with electric strip heat, a heat pump, or both. Heat pump models switch to strip heat in cold weather. Check settings, then the heater element and its limits, then the reversing valve.

5 steps ~60 minmoderate
Guide · Air Conditioner

PTAC error codes: how to read and clear them

Most PTAC brands store fault codes that can be shown with a button combination on the control (for example GE Zoneline uses a MODE + another key sequence on some models). Read the active code, fix the cause, then clear it.

4 steps ~20 mineasy

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Ductless mini-split heat pump / AC: field service manual

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Split heat pump: electrical basics field manual (reversing valve coil & defrost board) · OnCall Tech