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Heat Pump Defrost Cycle: Why Your Outdoor Unit Steams, Stops and Starts Again
-
By
Michael Haines
- Sep 12, 2026
What a defrost cycle is, how long it should run, and how to tell normal winter frost from a unit that needs a technician.
Last updated September 12, 2026 | By Michael Haines, Founder, AC Direct
A heat pump defrost cycle is a short, automatic reversal that sends warm refrigerant through the outdoor coil to melt frost. Steam rising from the unit, a pause in the outdoor fan and a whooshing sound during these periodic defrosts, which typically last 5 to 15 minutes, are normal in cold, damp weather.
- Frost forms because the outdoor coil must run colder than the outdoor air to absorb heat from it.
- During defrost the system runs in cooling mode briefly, the outdoor fan stops or slows, and the coil warms.
- Steam, dripping water and a swish or whoosh at the start and end are expected.
- A coil encased in solid ice, a unit stuck in defrost, or backup heat that never shuts off calls for a technician.
- Placement matters: keep the unit clear of roof runoff, snow and walkways where melt water can refreeze.
What is a heat pump defrost cycle?
A heat pump defrost cycle is a control sequence that clears frost from the outdoor coil. The system briefly switches to cooling mode, so hot refrigerant flows through the outdoor coil instead of the indoor coil, melts the frost, and then returns to heating once the coil is clear.
The Consortium for Energy Efficiency describes it plainly: when outdoor coils drop below 32°F, condensation freezes on them and forms frost, and the heat pump periodically enters a defrost cycle that switches to cooling mode and melts the ice with warm refrigerant. Fans might stop or slow, and auxiliary heating could activate to keep the indoor temperature stable.
Think of the rear window defroster on a car. Nobody considers the car broken because the glass fogs over on a January morning. The defroster is part of the design, and so is this. For how defrost fits alongside capacity ratings and backup heat, see our hub on inverter heat pumps in cold climates.
Why does frost form on a heat pump in winter?
Frost forms because a heat pump's outdoor coil must be colder than the outdoor air to pull heat from it. When the coil surface is below both freezing and the dew point of the air, moisture in the air freezes onto the fins. Cold, humid weather near or just above freezing builds frost fastest.
Oak Ridge National Laboratory researchers explain that when the heat exchanger surface is below freezing and below the dew point of the air, moisture freezes on the surface and frost starts growing. The frost hinders airflow, and the lower airflow and added thermal resistance reduce the heat pump's heating capacity and efficiency. Once performance falls to a minimum acceptable level, a defrost process removes the frost layer.
A glass of iced tea on a humid porch shows the first half of this. Warm, moist air meets a cold surface and condensation forms, as the EPA puts it. Drop the glass below 32°F and the droplets turn to frost. Your outdoor coil is a very large glass of iced tea that nobody asked for.
Humidity drives the schedule. The same Oak Ridge review notes that ambient relative humidity greatly affects the amount of frost formation and is directly related to how often defrost is needed. That is why defrost seems to happen on the dreariest days rather than the coldest ones.
How does the defrost cycle work?
The defrost cycle works by shifting the reversing valve so the heat pump runs in cooling mode. Hot refrigerant from the compressor flows to the outdoor coil and melts the frost, the outdoor fan stops so it does not blow cold air across the coil, and the controls switch back to heating when the coil is clear.
Reverse cycle defrost needs little extra hardware: the Oak Ridge review notes that, besides a four-way valve, it does not require complicated components. The heat for it comes partly from the indoor air, which can affect indoor comfort.
Here's what that means for the people inside. While the outdoor coil is borrowing heat, something has to cover the house. The CEE guide says that in ducted systems supplemental heat may come on during defrost to prevent a gap in heating, while a ductless indoor head may put off some cooling while the outdoor coil warms. Our article on heat pump auxiliary heat explains why that AUX light appears.
The outdoor fan stops for a sensible reason. Blowing 20°F air across a coil you are trying to warm would be like opening the oven door to speed up dinner.
How long and how often should defrost run?
A normal defrost typically lasts 5 to 15 minutes. Frequency depends on outdoor temperature, humidity and the unit's controls, so a heat pump may defrost several times on a damp day near freezing and rarely on a dry, frigid one. A unit that stays in defrost much longer, or never clears, needs attention.
Defrost takes a small share of total run time, even if it looks dramatic. In NREL's cold climate field study, all systems spent a relatively small fraction of operating hours in defrost, but defrost energy exceeded 20% of compressor-based heating energy at 4 of 12 sites because the auxiliary heater ran during defrost. Two sites ran the auxiliary heater for more than five minutes after the defrost cycle ended.
The catch is that the cost of defrost is mostly the backup heat around it, not the defrost itself.
Controls decide the frequency. The Oak Ridge defrost review, discussing commercial refrigeration, explains that a time-based defrosting strategy is determined from worst case conditions, so unnecessary defrost cycles will likely occur. Demand defrost strategies, which respond to actual frost, have been investigated in applications including heat pumps.
How do inverter heat pumps handle defrost differently?
Inverter heat pumps still defrost, because frost physics does not care about compressor type. The difference is control: a variable-speed compressor can adjust its speed around a defrost, and the controls, whether timed or demand-based, decide when defrost starts and ends.
Cold-climate testing treats defrost as part of real operation. The ENERGY STAR Cold Climate Heat Pump Controls Verification Procedure runs a defrost cycle, terminates it with the heat pump's own defrost controls, and starts the official test period 10 minutes after defrost ends. A unit has to recover from its own defrost and still meet the 5°F requirements.
An inverter does not get a pass on frost. It gets a better plan for the minutes after it. After defrost, a variable-speed compressor can run faster to replace the heat the house lent to the coil, a flexibility fixed-speed designs do not have and one of the practical advantages of inverter systems in cold, damp climates. See how heat pumps work in very cold weather.
What does a normal defrost look and sound like?
A normal defrost looks like steam or vapor rising from the outdoor unit, the outdoor fan stopping while the compressor keeps running, and water dripping from the coil. It sounds like a swish or whoosh when refrigerant flow reverses at the start and end, with the fan restarting once heating resumes.
Steam is melting frost meeting cold air, not smoke. The NEEP consumer guide notes that it is normal to hear occasional gurgling or swishing sounds when a ductless indoor unit is defrosting. The water has to go somewhere, and ENERGY STAR advises that outdoor units release water when they defrost, which can form icy patches, so avoid installing them near doors and windows.
| What you see or hear | What it means |
|---|---|
| Steam rising from the outdoor unit for several minutes | Normal: frost melting on a warmed coil |
| Outdoor fan stops while the compressor keeps running | Normal: the unit is in defrost mode |
| Swish or whoosh at the start and end of the cycle | Normal: refrigerant flow reversing |
| Water dripping or pooling under the unit | Normal: melt water draining, but watch for ice on walkways |
| Thin, even frost that clears after each cycle | Normal: routine frost in cold, damp weather |
| Coil or cabinet encased in solid ice that does not clear | Problem: defrost is not keeping up; call a technician |
| Unit stuck in defrost well beyond 15 minutes | Problem: controls or sensors may need service |
| AUX constantly displayed on the thermostat | Problem: backup heat carrying the house; have the system checked |
| Roof runoff dripping onto the unit and freezing | Problem: placement; ask a contractor about a snow deflector or relocation |
When is ice on a heat pump a problem?
Ice on a heat pump is a problem when it does not clear after a defrost cycle, when it builds into a solid block over the coil or top of the cabinet, or when the unit stays in defrost far longer than 15 minutes. At that point the heat pump cannot move air through the coil, and backup heat carries the house.
The CEE guide says that if a heat pump is stuck in defrost or covered in ice, consult an HVAC professional, and it lists frozen coils and auxiliary heating constantly displayed on the thermostat among the signs a system is not working properly. The same guide warns against placing a unit under a roof eave's drip line and suggests a snow stand in cold climates.
In practice, the most common causes of a unit encased in ice are outside the machine: gutter overflow, drifting snow and a unit sitting too low to drain. Hot water from a kettle is not a repair. Water poured on a frozen unit refreezes, and then there is more ice than before.
Picture identical heat pumps on two homes. One sits on a stand at the gable end. The other sits under a roof valley where meltwater drips all afternoon and freezes all night. The first defrosts and goes back to work. The second grows a new layer of ice every evening, because it has its own private storm. For what a hard stretch of weather can do to a system, read how one heat pump struggled through a March cold snap.
Steam, a stopped fan, a whoosh and a puddle are a heat pump taking care of itself. Solid ice that stays, a unit stuck in defrost, or backup heat that never turns off are the signs to call a professional.
Your outdoor unit is not on fire. It is taking a sauna break, and it will be back at work in about ten minutes.
Frequently Asked Questions
Is it normal for a heat pump to steam in winter?
Yes, steam from a heat pump's outdoor unit in winter is normal during a defrost cycle. The system warms the outdoor coil to melt frost, and the melting frost releases vapor into the cold air. Steam that stops within about 15 minutes, followed by the outdoor fan restarting, indicates a routine defrost.
How long does a heat pump defrost cycle last?
A heat pump defrost cycle typically lasts 5 to 15 minutes. The exact time depends on how much frost has formed, the outdoor temperature and the unit's controls. A heat pump that stays in defrost well beyond 15 minutes, or repeats defrost without clearing the coil, should be checked by a qualified technician.
How often does a heat pump go into defrost?
A heat pump goes into defrost as often as frost builds on the outdoor coil, which depends mostly on humidity and temperature. Damp weather near freezing causes the most frequent defrosts, while dry, very cold air produces less frost. Units with demand defrost controls respond to actual frost instead of a fixed timer.
Why does my heat pump blow cool air during defrost?
A heat pump can blow cooler air during defrost because it temporarily runs in cooling mode to warm the outdoor coil. Ducted systems may turn on backup heat to cover that gap, while ductless indoor units may release some cooler air. Normal heating returns when the defrost cycle ends and the coil is clear.
Should I pour hot water on a frozen heat pump?
Pouring hot water on a frozen heat pump is not recommended, because the water can refreeze on the coil, base and nearby ground. Clear loose snow away from the unit, check for roof runoff dripping onto it, and call a qualified HVAC technician if the coil stays encased in ice after a defrost cycle.
Can I cover my heat pump in winter?
A heat pump should not be covered in winter, because it needs open airflow across the outdoor coil to heat the home and to defrost. Keep snow and debris clear, and protect the unit from roof runoff with proper placement, a snow stand or a snow deflector recommended by a qualified contractor.
