Heat Pump Steaming, Whooshing or Blowing Cool Air? Defrost Mode Explained
Steam from the outdoor unit, a whoosh, the fan stopping, cool air for a few minutes.
By Mark Curant·Updated
Most likely causes
- Normal defrost: the heat pump briefly runs in reverse to melt frost
- Defrosting very often because of snow, dirt or poor airflow
- A defrost cycle that doesn't end: a failed board or sensor
- Backup heat not coming on during defrost, so the indoor air feels cold
Before you open anything
Turn the system off at the thermostat, then switch off power at the breaker and at the disconnect beside the outdoor unit. Never touch the capacitor, even with the power off: it can hold a dangerous charge. Refrigerant work is for licensed technicians only. Reset a tripped breaker once; if it trips again, leave it off.
Check these first (free, 5 minutes)
- Time it: a normal defrost lasts about 2 to 10 minutes, up to about 15 in heavy frost.
- Count them: in freezing, damp weather, defrosts every 30 to 90 minutes are normal. In mild, dry weather they should be rare.
- Watch the outdoor fan: on most models it stops during defrost and starts again afterwards.
- Look at the coil afterwards: it should be clear of frost.
Causes, signs and repair costs
Normal defrost cycle
DIYWhoosh, fan stops, steam for a few minutes, then normal heating resumes
Typical cost: $0 (normal)
Dirty coil or blocked airflow
DIYDefrosts more often than usual; leaves or snow against the unit; a dirty indoor filter
Typical cost: $0–$20
Backup heat not working during defrost
ProIndoor air is noticeably cold every time it defrosts
Typical cost: $75–$200 service call plus parts
Failed defrost board or sensor
ProDefrost cycles back to back, last well over 15 minutes, or never happen
Typical cost: $200–$650
Reversing valve or refrigerant problem
ProDefrost runs but doesn't clear the coil; heating weak afterwards
Typical cost: $350–$1,200 valve, $400–$2,400 leak repair
| Cause | Signs | Who fixes it | Typical cost |
|---|---|---|---|
| Normal defrost cycle | Whoosh, fan stops, steam for a few minutes, then normal heating resumes | DIY | $0 (normal) |
| Dirty coil or blocked airflow | Defrosts more often than usual; leaves or snow against the unit; a dirty indoor filter | DIY | $0–$20 |
| Backup heat not working during defrost | Indoor air is noticeably cold every time it defrosts | Pro | $75–$200 service call plus parts |
| Failed defrost board or sensor | Defrost cycles back to back, last well over 15 minutes, or never happen | Pro | $200–$650 |
| Reversing valve or refrigerant problem | Defrost runs but doesn't clear the coil; heating weak afterwards | Pro | $350–$1,200 valve, $400–$2,400 leak repair |
Typical U.S. installed prices in 2026. Most repairs also carry a service call fee of $75–$200, often credited toward the repair. See AC repair costs in 2026 for the full list.
Step by step
- 1
Let a cycle finish
Don't switch the system off when you see steam. A defrost is the heat pump briefly running as an air conditioner so warm refrigerant melts frost off the outdoor coil. The steam is that melt water evaporating. Interrupting it leaves ice behind.
- 2
Keep the outdoor unit clear
Snow, leaves and dirt on the coil make frost build faster, so the unit defrosts more often. Keep 2 feet clear around it. In mild weather, a gentle rinse of the coil with the power off helps.
- 3
Check the filter indoors
A clogged filter makes the whole system work harder in heating, too. Replace it if it's gray.
- 4
Note what's not normal
Write down how long each defrost lasts and how often it happens. Cycles over 15 minutes, back-to-back cycles in dry weather, or no defrost at all while ice builds up are worth a service call, and the notes help the technician.
Call a technician if
- Defrost cycles last much longer than 15 minutes or run back to back
- Ice builds up and the unit never seems to defrost
- You hear grinding or screeching, or smell burning (switch off and call)
- The house gets cold every time it defrosts
Red flags on a quote
- A new system recommended because the unit "steams": steam during defrost is normal
- A defrost board replaced without testing the coil sensor first
Facing an expensive repair on an older system? Run it through the repair vs replace calculator, or read the $5,000 rule.
Frequently asked questions
Why is steam coming out of my heat pump?
It's almost always the defrost cycle. In cold weather, frost forms on the outdoor coil, and the heat pump periodically reverses to warm the coil and melt it. The melt water evaporates off the warm coil as steam. It's normal and stops within a few minutes.
How often should a heat pump go into defrost?
It depends on the weather and the controls. In freezing, damp weather, every 30 to 90 minutes is normal; in dry or milder weather it may rarely defrost at all. Each cycle usually lasts 2 to 10 minutes.
Why does my heat pump blow cold air during defrost?
During defrost the heat pump is moving heat to the outdoor coil, so it isn't heating the house for those few minutes. Most systems switch on backup heat to temper the air. If the air is noticeably cold every time, the backup heat may not be working.
What is the loud whoosh from my heat pump?
That's the reversing valve switching the direction of the refrigerant at the start and end of a defrost cycle. A short whoosh or hiss is normal. Grinding, screeching or banging isn't.
Related problems
Heat pump iced up
Frost or thick ice on the outdoor unit that doesn't clear.
Heat pump not heating
It runs in winter, but the house stays cold or the vents blow cool air.
AC making noise
Buzzing, clicking, hissing, banging, squealing or rattling from the indoor or outdoor unit.
Fixed it? Check what it costs to run
If your system is old enough to keep breaking, compare its running cost with a new one before the next repair.
Open the SEER savings calculator