On a cold, damp Spokane morning it is normal to look out and see a heat pump coated in white frost, or puffing what looks like smoke. Heat pumps are built to frost up and clear themselves many times a day. What is not normal is a unit that stays encased in ice, a fan scraping against ice, or a house that has been running on backup heat for days.
Safety first: Never chip, pry or scrape ice off a heat pump with a shovel, screwdriver, ice scraper, hammer or any other tool. The coil fins are thin aluminum, and the tubes behind them carry refrigerant under pressure. One slip punctures a tube, and a frozen unit becomes a sealed-system repair. Do not pour hot water on it either: on a freezing day the runoff refreezes in the base and around the feet. Keep hands away from the fan grille, because the fan can start on its own when a defrost cycle ends. Turn the system off at the thermostat before you clear snow near the unit.
Why a heat pump frosts up
In heating mode, the outdoor coil is the cold side of the system. Refrigerant runs through it colder than the outdoor air so it can pull heat out of that air and carry it inside. When the coil surface is below freezing, moisture in the air freezes onto the fins, the same way frost forms on a windshield overnight. Spokane winters are good at this. During a winter inversion the city can sit under cold, damp air and freezing fog for days, and a heat pump breathing that air frosts quickly.
Frost blocks airflow across the coil, so the heat pump collects less heat. The defrost control watches for that and clears it.
What normal defrost looks and sounds like
When the control decides the coil needs it, the heat pump briefly switches into cooling mode, so hot refrigerant flows through the outdoor coil and melts the frost from the inside. What you will notice:
- The outdoor fan stops while the compressor keeps running. Stopping the fan keeps the heat in the coil.
- A loud whoosh as the reversing valve shifts the refrigerant flow, sometimes with a clunk. You hear it again when defrost ends.
- Steam rising off the unit. A warm, wet coil in cold air makes a cloud that can look like smoke. It is water vapor.
- Water running out the bottom as the frost melts.
- Inside, the thermostat may show AUX while backup heat runs so the air from the registers does not turn cool.
A defrost cycle usually lasts a few minutes. The control ends it once the coil has warmed up, or after a time limit even if a little frost is left. Then the valve whooshes back, the fan restarts, and heating resumes. How often it happens depends on the weather and the control. Older timer-based boards check the coil every 30, 60 or 90 minutes of run time; newer demand-defrost controls start a cycle only when sensors say the coil is losing efficiency. Damp, near-freezing weather brings frequent defrosts. Very cold, dry air often brings fewer.
After a normal defrost the fins are visible again, and that frost-and-clear rhythm can repeat all day in a cold snap. That is the heat pump working as designed.
Signs it is a real problem
- A solid block of ice. You cannot see the fins at all, the coil is encased in ice, and it has stayed that way for hours, through what should have been several defrost cycles.
- Fan blades hitting ice. A scraping, ticking or grinding sound from the top means ice has built up under the fan grille or up from the base into the blades. Set the thermostat to AUX or EM heat right away so the outdoor unit stops.
- Ice on top of the unit. A cap of ice on the top grille, or icicles hanging off it, usually means water is dripping onto the unit from a roof edge or gutter. Defrost is built to melt frost on the coil, not a steady drip from above.
- Ice growing from the bottom up. Meltwater that cannot drain away refreezes in the base pan and around the unit and climbs into the lower coil.
- Frost on only part of the coil, such as the bottom rows, while the rest stays clear. That can point to a refrigerant problem.
- No defrost at all. The frost keeps thickening, and you never see the fan stop or hear the whoosh.
- Days on backup heat. The thermostat shows AUX almost constantly, even in mild weather.
Homeowner checks
- Give it one defrost cycle. If the unit is frosted but running, check back over the next hour or so. If you see the fan stop, hear the whoosh, see steam, and the fins come back into view, it is fine.
- Clear snow and drifts. With the system off at the thermostat, brush loose snow off the top and away from all sides with a soft broom or a gloved hand. A heat pump pulls air in through its sides and blows it out the top, so it needs open space all around, plus a path for meltwater to drain away underneath. Watch for snow sliding off the roof, shovel piles, and wind-driven drifts against the house, which can bury a unit overnight. Leave any ice stuck to the coil alone, and turn the system back on when you are done.
- Look up. Check what is directly above the unit. An overflowing or ice-dammed gutter, a leaking gutter seam, or a roof edge with no gutter can drip on the unit all afternoon as snow melts in the sun, and the drip freezes when the temperature drops at night. The lasting fix is to stop the water landing there: clear or repair the gutter when it is safe to do so, add a diverter, or move the unit. Do not climb onto an icy roof to do it.
- Check that the unit sits up off the ground. In snow country an outdoor unit should sit on a riser stand or wall brackets that hold it above typical snow depth and let meltwater drain away. A unit on a pad at ground level can end up sitting in its own refrozen meltwater, with ice building from the base after every defrost. Raising a low unit is part of our heat pump installation work.
- Check indoor airflow. With the system off, check the filter in the air handler or furnace and replace it if it is dirty. Open supply registers and clear return grilles. During defrost, the heat that melts the outdoor ice comes from indoor air passing over the indoor coil. With weak airflow there is less heat to work with, defrost can end before the coil is clear, and the ice gains a little each round. Poor airflow also makes the heat pump work harder and can trip its high-pressure safety.
- Read the thermostat. AUX showing during a cold snap or during defrost is normal. If someone switched the thermostat to AUX or EM heat and forgot, the heat pump is not running at all.
- Check power. If the outdoor unit does not run at all while the thermostat calls for heat, check whether the heat pump breaker has tripped and reset it once; if it trips again, leave it off and call us.
If the coil is a solid block of ice or the fan is hitting ice, set the thermostat to AUX or EM heat and leave it there. That stops the outdoor unit while backup heat keeps the house warm. Let the ice melt on its own, keep snow cleared around the unit, and call us. Running a heat pump encased in ice can damage the fan, the motor and, over time, the compressor.
AUX or EM heat: what it does
A heat pump gathers less heat as the outdoor air gets colder, so most systems have backup heat. In an all-electric system it is a set of electric heat strips in the indoor air handler. In a dual-fuel system the backup is a gas furnace.
When the thermostat brings in backup heat on its own, alongside the heat pump, the display usually shows AUX. That happens when the heat pump cannot keep up, when you raise the setpoint several degrees at once, and during defrost. Seeing it in a cold snap is expected.
The setting you choose yourself is labeled EM heat on many thermostats and something like AUX heat only on others. Either way it turns the heat pump off completely and heats the house on backup alone. Use it when the outdoor unit is iced solid, damaged or noisy, to protect it until a technician looks.
Treat it as a bridge, not a fix. Electric heat strips use far more electricity than a working heat pump to deliver the same heat, so bills climb quickly, and whatever caused the ice is still there. If you have been running on AUX or EM heat for days, or the thermostat shows AUX nearly all the time even in mild weather, it is time to call. Some heat pumps are set up to shut off below a chosen outdoor temperature and let backup heat take over; that is a setup choice, not a failure.
Defrost sensors and boards: the technician side
When a heat pump ices up and snow, drips and airflow are not to blame, we look at the parts that run defrost and at the refrigerant side. This is technician work.
- Defrost control board. It decides when defrost starts and ends. A failed board or relay may never start a defrost, or may start one without stopping the outdoor fan.
- Coil sensor. A small sensor clipped to a tube near the bottom of the outdoor coil tells the board how cold the coil is. If it fails, reads wrong or slips off its tube, the board may skip defrost or end it too early.
- Outdoor air sensor. Demand-defrost systems compare coil and outdoor temperatures, so a bad outdoor reading throws off the decision.
- Reversing valve. If the valve or its solenoid sticks, the system cannot switch into defrost at all.
- Outdoor fan motor and capacitor. A fan that runs slowly or not at all moves too little air across the coil, and the coil ices fast.
- Refrigerant charge. A system low on refrigerant runs the coil colder than it should, often frosting one section heavily, and defrost cannot keep up. Low charge means a leak, which we find and repair before recharging. Refrigerant is never a homeowner job.
We also use the board's test function to force a defrost and watch the whole sequence: whether the valve shifts, the fan stops, the backup heat comes on, and the coil clears.
Ductless mini-splits
Ductless mini-split heat pumps frost and defrost the same way. During defrost the indoor head usually stops blowing so it does not push cold air into the room, and a light on the head may blink. That pause is normal. Outdoor units should sit on wall brackets or a tall stand above the snow, and watch the drain under the unit, which can freeze into a column of ice that reaches the base. If the outdoor unit is iced solid, or the head stays stopped for a long time, turn it off with the remote and call.
What to tell the office when you call
- Model numbers from the data plate on the side of the outdoor unit and from the indoor air handler or furnace. A photo works.
- Photos of the ice: whole coil, bottom only, or a cap on top, and how thick.
- How long it has been iced and whether you have seen a defrost cycle.
- Sounds: fan scraping, no whoosh, or fan not running.
- What the thermostat shows, and how long you have been on AUX or EM heat.
- Your backup heat: electric strips, a gas furnace, or none, as with many mini-splits.
- When it started and what came before: a snowstorm, a thaw and refreeze, a gutter that started overflowing.
- What you already tried: snow cleared, filter changed, breaker reset once.
If you ask about cost, the office names the diagnostic fee when you call, and the technician writes one price before any work starts. You can read more about our heat pump repair in Spokane.
When to call us
- The coil is a solid block of ice that has not cleared after several hours.
- The fan is hitting ice, or the unit grinds or scrapes.
- You never see a defrost cycle, or the fan keeps running while the coil ices.
- Only part of the coil frosts heavily.
- You have been on AUX or EM heat for days, or AUX is on almost all the time.
- The breaker trips a second time.
- Roof or gutter water keeps icing the unit, or it sits too low and needs to be raised.
We answer calls every day; call and the office tells you whether a technician can reach your address today. You can also call or text (509) 370-5361 and send photos of the ice.
Page reviewed by Adam, Larch technician
Technician · heating, cooling and appliances · Updated October 2026
Adam works on furnaces, heat pumps, AC and household appliances on our Spokane-area calls. He tests first, shows you the failed part and writes the price before any repair.
