Why is my heat pump steaming and making noise in winter?
Your heat pump is almost certainly running a defrost cycle, reversing for a few minutes to melt frost off the outdoor coil. The outdoor fan stops, steam rises, the reversing valve whooshes and water drips out the bottom. That's normal. A coil that stays encased in ice afterward is not.
Key points
- Steam, a stopped outdoor fan and a whoosh at the start and end are a normal defrost cycle.
- Defrost is most frequent on damp days in the 20s and 30s, and less frequent in dry, deep cold.
- Indoors, backup heat or the furnace often runs briefly; a ductless head's fan stops until the coil warms.
- A coil that stays iced over, or ice building up under the unit, needs attention.
- Most ice problems trace back to where the unit sits and where its meltwater goes.
Steam, a loud whoosh and an outdoor fan that stops in winter almost always mean your heat pump is defrosting. In heating mode, frost builds on the outdoor coil, so every so often the system reverses for a few minutes to melt it off, then goes back to heating. It's normal, and on damp days in the 20s and 30s it happens often. What isn't normal is a coil that stays encased in ice, ice building up under the unit, or a system that seems to defrost more than it heats.
What does a heat pump defrost cycle look and sound like?
In heating mode, the outdoor coil runs colder than the outside air, so moisture freezes onto it. When enough frost builds up, the system reverses for a few minutes to melt it off:
- A whoosh. The reversing valve shifts and sends hot refrigerant to the outdoor coil.
- The outdoor fan stops, so the heat stays in the coil.
- Steam rises as the frost melts. On a still morning it can look like smoke.
- Water drips out the bottom of the unit.
- Another whoosh as the valve shifts back, and the fan starts again.
The whole thing usually takes a few minutes. Steam disappears within seconds and has no smell. Smoke hangs in the air and smells like something burning; if you see that, turn the system off at the thermostat and call.
Why does a heat pump defrost most on damp days?
Frost forms fastest when the air holds plenty of moisture and the coil runs below freezing, which in Minnesota means foggy, drizzly days from the 20s into the 30s. On a dry, clear -10°F night there is little moisture left in the air, so the unit can go much longer between defrosts. Our post on heat pumps at -20°F covers what else changes in deep cold.
How often it defrosts also depends on the control. Older boards defrost on a timer, often at an interval the installer picks (30, 60 or 90 minutes are common settings), whether or not much frost has formed. Newer demand-defrost controls use coil and outdoor temperature sensors and defrost only when needed. Daikin calls its version Intelligent Defrost Mode on the FIT, and Mitsubishi says its Advanced Frost Detection reduces the number of defrost cycles.
What happens inside the house during a heat pump defrost?
For those few minutes the system is technically in cooling mode, so what you notice depends on the setup:
- Ducted heat pump with electric backup. The heat strips usually come on to warm the air. Daikin's FIT has a selectable defrost backup heat option; turning it off saves power during defrost, at the cost of cooler air for a few minutes.
- Dual fuel. Many controls fire the furnace briefly, so the registers keep putting out warm air. Our dual-fuel guide explains the setup.
- Ductless mini-split. The indoor fan stops so the head doesn't blow cold air, and the head goes quiet for a few minutes. Features such as Mitsubishi's Hot-start and Daikin's hot start technology hold the fan until the coil is warm again. See our ductless mini-split page for more on these systems.
If cool air from the registers is the part that worries you, our post on a furnace or heat pump blowing cold air covers the other causes.
Normal versus call for service
| What you see | Normal? | Likely cause if it isn't | What to do |
|---|---|---|---|
| Light, even frost that clears after each defrost | Yes | Nothing | |
| Steam for a few minutes, then the fan restarts | Yes | Nothing | |
| Water dripping under the unit | Yes | Keep the drain path clear | |
| Backup heat or the furnace running during defrost | Yes | Nothing | |
| Coil still encased in ice after a defrost | No | A defrost sensor or board fault, low refrigerant or a sticking reversing valve | Switch a ducted system to emergency heat and call |
| Defrosting over and over with little heat in between | No | Defrost control or sensor, or low refrigerant | Call |
| Ice building up under the unit toward the coil | No | Meltwater can't drain: unit too low, drain holes frozen, a drip from above | Clear snow, fix the drip, call if ice reaches the coil |
| Grinding or rattling | No | The fan blade hitting ice | Turn the system off and call |
| Backup heat running constantly on a mild day | No | The heat pump isn't carrying the load | Call |
Why does ice build up under a heat pump in Minnesota?
Most ice problems start with where the unit sits, not with the heat pump itself. Defrost water has to drain and get away before it refreezes. It can't when the unit sits on the ground in a drift, when its drain holes are frozen shut, or when a gutter drips on it all day during a thaw. Daikin's FIT installation manual calls for a stand that puts the bottom frame more than 19¾ inches above the expected snow, and it highly recommends a drain pan heater where the temperature stays below 32°F for more than 12 hours. Mitsubishi describes its freeze-prevention base pan heater as a safeguard for cold climates. We mount outdoor units on a stand or bracket above typical snow depth, out from under roof edges.
Minnesota winters test placement in a few particular ways. Freezing drizzle and ice storms can glaze the whole cabinet at once. Snow sliding off a steep roof can bury a unit set under the eave. A heat pump that went onto the ground pad left from an old air conditioner sits right where meltwater and drifting snow collect. And January thaws, followed by a hard freeze, turn a slow gutter drip into a column of ice on top of the fan. Most of these are fixed by moving the water or raising the unit, not by repairing the heat pump.
What you can do, and what we check
If the outdoor unit is icing up:
- Brush snow off the top and clear drifts away from the sides.
- Redirect a gutter or downspout that drips onto the unit.
- Leave ice on the coil alone. Don't chip it or pour hot water on it; the fins bend and the tubing can puncture.
- Watch the next defrost cycle. If the coil clears, you're done.
- If the coil stays iced solid and the house is losing heat, switch the thermostat to emergency heat so only the backup runs, and call.
Never cover a heat pump for winter the way some people cover an air conditioner, because it runs all winter. See covering an AC in winter.
When we come out, we check the defrost sensor and control board, the reversing valve, the outdoor fan motor and the refrigerant charge (a low charge makes the coil run colder and frost faster), and we look at whether placement and drainage are the real cause.
What a defrost problem costs to fix
A service call has a $79 trip charge, waived when you approve the repair, and you get a written, flat-rate price before any work starts. For scale, national cost guides such as HomeGuide put an outdoor (condenser) fan motor at $200–$700 and a refrigerant leak search and repair at $200–$1,500. When placement is the real cause, a stand, a bracket or a rerouted downspout is the fix, and it's priced in writing like any other repair. If the heat pump is old enough that replacement makes more sense, our installed range for a heat pump is $7,500–$14,500, and our heat pump installation cost guide explains what moves it. For more on how these systems handle our winters, see cold-climate heat pumps in Minnesota.
Call 612-429-3465, 7am to 9pm any day, or see our heat pump page. The price is the same for heat pump service in Minneapolis as in every other town we serve.
Questions
How long should a heat pump defrost cycle last?
Usually a few minutes. The outdoor fan stops, steam rises, and the fan restarts once the coil is clear. Light frost that's gone after the cycle is fine. If the unit seems to be defrosting most of the time, or the coil stays iced over after a cycle, have it checked.
How often should a heat pump defrost?
It depends on the weather and the control. On damp days in the 20s and 30s it may defrost fairly often; on dry, very cold days much less. Older timed controls defrost on a set interval, while demand-defrost controls go by sensors. A unit that barely heats between defrosts needs a look.
Is it normal for my mini-split to stop blowing in winter?
Yes, during defrost. The indoor fan stops so the head doesn't blow cold air while the outdoor unit melts its frost, and it starts again once the coil warms up. If the head stays off for long stretches or shows an error code, call for service.
Should I pour hot water on an iced-up heat pump?
No. The water can refreeze into worse ice, and chipping at the coil bends the fins and can puncture the tubing. Clear snow from around the unit, fix any drip from above, and let the defrost cycle work. If the coil stays solid with ice, switch to emergency heat and call.
Sources
- HomeGuide cost guides (2026 editions) homeguide.com