Heat pump defrost cycle
That burst of steam and cool air from your heat pump in winter is the defrost cycle doing its job. Here is what is happening.
What the defrost cycle does
In cold, damp weather, frost builds on the outdoor coil and blocks airflow. To clear it, the heat pump briefly reverses into cooling mode, sending hot refrigerant outside to melt the frost. During this time the outdoor unit steams and the indoor air goes cool for a few minutes — your backup/aux heat may switch on to offset it. Then it returns to heating.
How often and how long
Defrost runs as needed — anywhere from every 30–90 minutes in frosty conditions to rarely in dry cold. Each cycle typically lasts a few minutes (up to ~10–15). Modern units use "demand defrost" sensors to defrost only when necessary, saving energy.
When defrosting signals a problem
Trouble signs: the unit defrosts constantly, a thick block of ice never clears, or it gets stuck blowing cold air for a long time. These point to a defrost-control/sensor fault, low refrigerant, or a stuck reversing valve — call a technician. Also keep the outdoor unit clear of snow and ice so it can shed meltwater.
Frequently asked questions
Why does my heat pump steam in winter?
It is in a defrost cycle, melting frost off the outdoor coil. The steam is normal and lasts a few minutes.
How often should a heat pump defrost?
As needed — from every 30–90 minutes in frosty, damp weather to rarely in dry cold. Demand-defrost models only do it when sensors detect frost.
Is constant defrosting normal?
No — frequent or continuous defrosting, or ice that never clears, suggests a defrost-control fault, low refrigerant, or airflow problem. Have it checked.
Related
Sources & further reading
Educational guide, reviewed against US DOE & ENERGY STAR guidance and updated June 2026. Estimates only — not a substitute for a professional assessment or Manual J load calculation.