In heating mode, a heat pump's outdoor coil runs colder than the winter air — so frost on it is as normal as dew on grass. The machine knows this and periodically clears itself. The problem starts when clearing stops. Here's the line between healthy frost and a service call, and why the garden-hose fix you saw online belongs nowhere near the answer.
Healthy: the frost-and-clear rhythm
A thin, even white coating that builds during cold, damp hours and vanishes after a defrost cycle (that pause-hiss-steam-cloud performance every 30–90 minutes of frosty running). Damp Puget Sound air at 30–40°F is peak frost weather — our region's heat pumps defrost more often than a Montana unit ever will, and that's fine.
Broken: the ice sculpture
Thick, hard ice wrapping the coil, ice sheets on top of the unit, a fan scraping or stopped, airflow visibly choked. That means defrost isn't happening or can't keep up, and the shortlist is specific: the defrost sensor/thermostat misreading the coil, the defrost control board not initiating, the reversing valve failing to flip the cycle, a low refrigerant charge (undercharged coils run extra-cold and frost violently), or simple airflow strangulation — leaves packed in the coil, fencing too close, or the unit sunk in a snowdrift after a rare lowland dump.
What to do — and the one thing not to
Switch the thermostat to its auxiliary (backup) heat setting (the house stays warm on the strips while the outdoor unit rests), clear debris from around the unit, and let any accessible ice melt on its own. Do not chip ice off the coil — those aluminum fins bend with a hard look, and a punctured coil turns a sensor-priced repair into a refrigerant-circuit one. The hose trick (rinsing with water) sometimes clears ice and sometimes builds a bigger, harder ice block an hour later; at most it treats the symptom while the cause keeps failing.
The visit
We watch a defrost initiate (or refuse to), test the sensor and board, verify the reversing valve, and gauge the charge — the five suspects separate cleanly with instruments. Most defrost repairs are single-visit with a written quote, and your power bill will thank you: a heat pump fighting through an ice blanket burns aux heat like a bonfire.