Defrost Cycle
A heat pump's routine for melting frost off its outdoor coil: the system briefly reverses into cooling mode, sending hot refrigerant outside until the coil clears. Steam, a whoosh at changeover, and a few minutes of cooler indoor air are all normal defrost behavior.
The longer answer
Frost forms because a heat pump's outdoor coil runs colder than the outdoor air in heating mode — in humid cold, moisture freezes onto it and gradually blocks airflow. Modern controls defrost on demand (when sensors detect frost is actually hurting performance) rather than on a fixed timer.
What's not normal: a unit encased in thick ice for days, frost climbing the refrigerant lines, or defrost running constantly. Those point to a stuck reversing valve, low charge, failed sensors, or drainage problems under the unit — diagnostic territory, and never a chip-the-ice-off situation (coil fins and refrigerant lines damage easily).
The Seattle angle
Puget Sound's damp 30s-and-40s winters are prime frost weather — local heat pumps defrost often, and that's the design working, not failing.
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