DIAGRAMS.INK / HVAC TRAINING

Why a heat pump runs backwards to melt its own frost

In cold, humid air the outdoor coil ices up. Step through the cycle and watch the refrigerant reverse, the outdoor fan stop and the indoor aux heat step in.

Hot refrigerant gasCold low-pressure refrigerantFrost on the outdoor coilCutaway teaching model, not to scale. Numbers: 1 outdoor coil, 2 outdoor fan, 3 reversing valve, 4 compressor, 5 indoor coil, 6 aux heat strips, 7 defrost control board.

Indoor aux heat during defrost

Outdoor temperature −2 °CVery coldMild Outdoor humidity 90 %DryHumid

Coil frost0%of outdoor coil blocked

Heat pump output100%vs. a clean coil

Outdoor fanRunningValve: heating position

Aux heat stripsOffelectric resistance

Supply airWarminto the house

A simplified model

Exact control sequences vary by manufacturer. Many units start defrost from a timer plus a coil-temperature or pressure check and end it on coil temperature or a time limit; fan, valve and aux-heat timing differ. This simulation shows one generic sequence for learning. Follow the unit's own service documentation for real equipment.