The Heat Pump Defrost Cycle: Why Your Outdoor Unit Steams on Cold Mornings
On a cold Las Vegas winter morning, homeowners with heat pumps sometimes look outside and see what appears to be smoke or steam rising off the outdoor unit, accompanied by the system making unfamiliar sounds and briefly blowing cool air indoors. It looks like something is going badly wrong. In most cases, it's the defrost cycle doing exactly what it's designed to do.
Why heat pumps develop frost at all
In heating mode, a heat pump reverses the process it uses for cooling: instead of removing heat from inside and dumping it outside, it extracts heat from outdoor air and moves it inside. To do that, the refrigerant in the outdoor coil has to be colder than the outside air so heat flows into it. When outdoor temperatures are low and there's moisture in the air, that very cold coil surface causes moisture to condense and freeze on the coil — frost.
Frost accumulation is a problem because it insulates the coil and blocks airflow, both of which reduce the system's ability to extract heat. Left unaddressed, a heavily frosted coil dramatically reduces heating capacity.
How the defrost cycle solves it
Heat pumps handle this automatically. When the system detects frost accumulation — via temperature sensors, timers, or in modern systems, demand-based sensing — it temporarily reverses into cooling mode. This sends hot refrigerant out to the outdoor coil, melting the frost. The outdoor fan typically stops during this cycle so the heat stays concentrated at the coil. Simultaneously, the system usually engages auxiliary electric heat indoors so you're not blasted with cold air while the outdoor unit is in defrost.
The whole cycle typically lasts a few minutes, then the system switches back to normal heating operation.
What's normal during defrost
- Steam or vapor rising from the outdoor unit — this is melted frost evaporating off a warm coil in cold air. It looks like smoke. It isn't.
- A whooshing or hissing sound — the reversing valve changing direction produces a distinct sound
- The outdoor fan stopping while the compressor keeps running — intentional, so heat concentrates at the coil
- Briefly cooler air from indoor vents — some systems allow a moment of this during the transition
- Water dripping or pooling under the outdoor unit — melted frost has to go somewhere
- A "defrost" indicator light on the thermostat or unit — many systems display this
How often defrost should run
In Las Vegas, defrost cycles are relatively infrequent compared to humid or genuinely cold climates, because our low humidity means less moisture available to freeze on the coil. On a typical Las Vegas winter morning where temperatures are in the 30s or 40s with low humidity, a heat pump may go through defrost occasionally or not at all. Defrost is more likely on the coldest mornings, or on the rare occasions when Las Vegas gets moisture in the air during a winter storm.
A system entering defrost very frequently — every 30 minutes or so, repeatedly — is worth investigating. That pattern suggests either a sensor problem, low refrigerant, restricted airflow, or a defrost control issue.
Signs of an actual problem
- Heavy ice buildup that doesn't clear — a thin frost layer that comes and goes is normal; a thick ice sheet encasing the coil that persists means defrost isn't working
- Ice on the top of the unit or on the fan blades — this suggests a different problem than normal coil frost, potentially a drainage issue or something more serious
- The system stuck in defrost — running in defrost continuously rather than cycling back to heating
- No defrost at all despite visible frost accumulation — a failed defrost board, sensor, or reversing valve
- Very frequent defrost cycling — as noted above
- Auxiliary heat running constantly — if the thermostat shows aux or emergency heat continuously, the heat pump isn't carrying the load it should
What not to do
Don't try to chip, scrape, or pry ice off the outdoor coil. The coil fins are thin aluminum and damage easily, and a punctured refrigerant line turns a minor problem into a major repair. Don't pour hot water on the unit either — it can refreeze and worsen the situation, and thermal shock isn't good for the equipment. If ice buildup is severe, turning the system off and letting it thaw, then calling for service, is the appropriate response.
Keeping the outdoor unit clear
The defrost cycle needs the melted water to drain away from the unit. Make sure the base of the outdoor unit isn't sitting in a depression where water pools, isn't blocked by landscaping debris, and has adequate clearance around it for airflow. In Las Vegas, gravel and desert landscaping that's migrated against the unit over time is a common and easily fixed obstruction.
Aspen Air services heat pumps across Las Vegas, Henderson, Summerlin, North Las Vegas, and the surrounding valley. Call (702) 612-2980.