Heat pump steam and lukewarm air: normal in cold weather
Trane's explanation: as the heat pump defrosts, a cloud of steam from the outdoor unit is the ice melting and evaporating, and is perfectly normal. Its demand defrost cycles usually last 3 to 5 minutes.
What is probably happening
- Frost builds on the outdoor coil in cold, damp weather. The unit periodically reverses to melt it. That is the steam, the whoosh, and the brief cool air at the vents.
- Heat pump supply air runs cooler than furnace air, so it can feel lukewarm on your hand while the house holds temperature.
- Your backup or auxiliary heat may come on during defrost. On the thermostat that shows as AUX or EM. Brief is fine.
Do this in the next five minutes
- Nothing to fix. Note how long the steam lasts. A few minutes is normal.
- Keep snow and ice cleared from around the outdoor unit and off the top. Do not chip ice off the coil.
- Leave the thermostat set. Big setbacks on a heat pump force the expensive backup heat on.
- Check the filter.
Do not
- Do not switch to EMERGENCY HEAT because the air feels cool. That runs on electric strips and costs far more.
- Do not pour hot water over the coil.
Who to call
No call needed
This is the machine working as designed. A seasonal maintenance visit is the right time to have the defrost control checked.
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What it should cost
$175 to $350
HVAC maintenance visit, national range, averaging $250
Source: HomeAdvisor, HVAC maintenance cost, updated June 2026. View source
National published range, not a quote for your job. The full cost table for this trade is on the hvac page.
Ask this contractor, on the phone, before anyone comes out
Tap each one as you ask it.
- At what outdoor temperature does my backup heat take over, and is that set correctly?
- Is the defrost control working on demand or on a timer?
- Is the refrigerant charge correct for heating mode?
End the call if
- A replacement pitch because 'heat pumps do not work in the cold'. The Department of Energy says cold climate models are designed to perform down to 5 degrees F.
- A charge for a repair on a unit that simply defrosted.