Symptom guide
Heat pump steaming: how manufacturers describe the defrost cycle
By the HeatAndAirCost editors · Last updated October 5, 2026 · How we research prices
What people commonly report
On a cold, damp morning, a cloud rises off the outdoor unit and people assume it is smoke. Often the fan has stopped and the unit sounds different. Some notice cooler air from the vents for a few minutes, or AUX on the thermostat. Trane’s summary of this exact scene is that the defrost cycle “may cause a cloud of steam to rise from the outdoor unit”[1]. If what you see is dark smoke, or you smell burning, that is not this page: go to burning smell.
What Trane and Lennox say is happening
- The outdoor coil runs colder than the air. Trane says the coil is typically 10 to 20 degrees F colder than the outdoor air in winter, so moisture condenses on it and freezes. That is why a defrost can start in the low 40s, though it is more common in the 20s and 30s[1].
- Frost blocks airflow. Lennox describes frost as acting like insulation on the coil, forcing the system to work harder for less output[2].
- The unit briefly reverses. The reversing valve switches the heat pump into cooling mode for a few minutes to warm the outdoor coil, then switches back[1]. The melting ice is the steam[2].
- Sounds change. Trane says you may hear a swooshing sound as refrigerant reverses, the outdoor fan may stop, and the compressor noise may rise or fall[1]. It also describes a hum while the fan is stopped during defrost[4].
How to tell a defrost cycle from a problem
- It ends on its own. Trane says its sensor-based systems usually finish in 3 to 5 minutes; Lennox describes the whole process as lasting a few minutes depending on weather and frost[1][2].
- The coil clears. After the cycle, the frost should be gone or mostly gone. Trane says ice that is not going away, or is growing, points to a bigger issue[3].
- It smells like nothing. Water vapor has no burning odor.
- The thermostat may show AUX. Trane says backup electric heat strips often keep the house warm during defrost[1].
Lennox adds that occasional defrosting is expected, but that “frequent or prolonged cycles may signal an issue that requires professional maintenance”[2]. DOE’s heat pump tip on the backup heat: avoid manually setting the thermostat back if doing so brings on the more expensive electric-resistance heat[5].
When to stop and call
- Ice stays or grows after a few hours. Trane lists low refrigerant, a faulty defrost control or a faulty sensor as possible causes[3].
- Defrost cycles seem to run constantly or last far longer than a few minutes.
- The cloud is dark, smells like burning, or comes with a loud new noise.
- The house is not staying warm.
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Sources
This page is general information collected from the official and manufacturer sources listed above. It is not an inspection or a diagnosis of your system, and only a qualified technician who sees the equipment can tell you what is wrong. HeatAndAirCost is not affiliated with the U.S. Department of Energy, Trane, Lennox. Found an error? hello@heatandaircost.com.
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People also ask
Heat pump steaming: FAQs
Why is my heat pump steaming?
Trane and Lennox both describe steam from the outdoor unit as part of the defrost cycle. The heat pump briefly reverses to warm its outdoor coil and melt frost, and the melting ice gives off water vapor. Trane says it can happen when it is in the low 40s outside, and is more common in the 20s and 30s.
How often does a heat pump go into defrost?
Lennox says it depends on conditions: on cold, damp days frost forms quickly and the cycle runs more often, while on dry or sunny days it may run rarely or not at all. Trane says its sensor-based systems only defrost when needed and usually finish in 3 to 5 minutes.
Why is cool air coming out of the vents during defrost?
During defrost the heat pump runs in cooling mode for a few minutes. Trane says most homes have backup electric heat that covers that time, often showing AUX on the thermostat, but homes without backup heat may feel cool air at the vents until the cycle ends.
When is ice on a heat pump a problem?
Trane says periodic frost on the outdoor coil is expected, but ice that is not going away or keeps growing points to a bigger issue such as low refrigerant, a faulty defrost control or a faulty sensor. Its advice is to wait a few hours to see if the unit clears itself, then call a professional if it does not.