Why is my heat pump steaming, whooshing, or icing up?
In cold, damp Puget Sound weather your heat pump's outdoor coil collects frost while it's absorbing heat from outdoor air; the unit periodically reverses into cooling mode for a few minutes to melt that frost — that's the defrost cycle. Steam rising off the outdoor unit, a whoosh at changeover, and briefly cooler supply air indoors are all normal. Ice that builds up and never clears is not — and in our climate, defrost behavior is something you'll notice more than homeowners in drier cold regions.
The quick answer
- Defrost is normal: the unit briefly reverses to melt frost off the outdoor coil
- Steam clouds and a whoosh sound during changeover are expected, especially 30–45°F and damp — i.e., Seattle
- Normal: light frost that comes and goes · Not normal: thick ice that builds for days
- Never chip ice off the coil — the fins and refrigerant lines damage easily
When to relax, when to call
Relax: visible steam for a few minutes, a swoosh or brief louder operation at the start and end of the cycle, water pooling then refreezing near the unit, and lukewarm indoor air for a few minutes while backup heat covers (if equipped).
Call: the outdoor unit encased in ice, frost climbing the refrigerant lines, defrost seeming to run constantly, or the top of the unit icing where the fan is — those point to a stuck reversing valve, low charge, a failed sensor, or drainage problems underneath.
Burien and West Seattle homes with heat pumps tucked beside the house on low ground see a specific pattern: defrost melt puddles, refreezes overnight, and builds an ice shelf that eventually engulfs the coil. That's drainage and placement, not a mysterious equipment defect — and it's preventable at install or with a raised pad.
What's happening during defrost
Heating mode makes the outdoor coil colder than outdoor air — that's how it absorbs heat. In Puget Sound's damp 30s and 40s, moisture in the air frosts onto that cold coil and gradually blocks airflow. The control board watches coil temperature and run conditions, and when frost is hurting performance it flips the reversing valve: for a few minutes the system runs as an air conditioner, pumping hot refrigerant through the outdoor coil to melt the frost, with the outdoor fan off. Then it flips back.
Modern units defrost on demand — only when sensors say frost is actually there. If yours seems to defrost on a rigid clock, constantly, or never, that's diagnostic information worth a visit.
Why placement and drainage matter here
Defrost water has to go somewhere. Units set too low, or on ground that puddles, refreeze their own melt water into an ice shelf that eventually reaches the coil. Good installs in our climate set the unit on a raised pad or brackets with drainage below — a small detail that prevents the most common winter ice complaints.
Annual maintenance verifies charge, defrost sensors, and reversing-valve operation before the damp season — because low refrigerant and bad sensors both masquerade as 'defrost problems' when the real issue is the unit never completing a proper melt cycle.
The ice mistakes that cause real damage
Chipping or prying ice off the coil bends fins and can puncture refrigerant lines — turning a service call into a repair. Pouring hot water works in an emergency thaw but refreezes into a worse problem underneath. And ignoring a fully iced unit forces the compressor to work against a blocked coil, which is how a $200 sensor problem becomes a compressor-scale one.
If the unit is iced solid: switch to emergency/backup heat if you have it, kill the ice at the source (gentle cold-water rinse, clear the drainage), and book a diagnostic. Don't run it iced for days.
Questions Seattle homeowners ask
How often should a heat pump defrost in Seattle weather?
In our classic cold-damp pattern (30s–40s and drizzly), every hour or two of runtime isn't unusual, for roughly 5–15 minutes at a time. Demand-defrost units do it less in dry cold. Constant defrosting, or none at all while frost visibly builds, both merit a look.
My vents blow cool air for a few minutes in winter — is it broken?
If it's brief and coincides with steam outside, that's defrost: the system is temporarily cooling indoors while it melts the coil, and backup heat (if equipped) tempers it. Cool air constantly, not in episodes, is a different problem — check the filter, then call.
Can I prevent frost entirely?
No — frost is physics in a damp climate. What you can control: airflow (keep leaves and shrubs off the coil, filters fresh), drainage under the unit, and annual maintenance that verifies charge and sensors, since low refrigerant and bad sensors both masquerade as 'defrost problems.' In Puget Sound's 30s–40s and drizzly pattern, seeing defrost a few times per hour of runtime is normal, not a sign your heat pump is struggling.
Should I worry if my neighbors' heat pump never steams?
Not necessarily — defrost frequency depends on humidity, coil temperature, run time, and control strategy. A unit in a drier microclimate or on a sunnier wall may defrost less. What matters is whether yours clears frost and returns to heating normally. Ice that persists for hours, or defrost that runs constantly without melting, is the comparison worth making — not whether you see steam and they don't.
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How we know this
Defrost behavior described per manufacturer service literature for the cold-climate equipment Glendale installs; failure patterns from our Puget Sound winter service calls.
Reviewed by the Glendale Heating & Air Conditioning team · Last updated 2026-07-15
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