Troubleshooting a heat pump
There's ice on the heat pump outside.
On a cold, damp morning, a little frost and a cloud of steam are part of how a heat pump works. Here's what's normal, and what isn't.
How to read this page
- NormalThe system doing its job. Nothing to do.
- Stop, and callTurn it off, or get out, then call.
Normal or not
What defrost looks like, and when ice is a problem
A heat pump melts its own ice with a defrost cycle. Most of what you see outside is that cycle working.
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Frost on the outdoor unit, even above freezing
In winter the outdoor coil is typically 10-20°F colder than the air around it, so it can frost over in the low 40s outside. Periodic frost is normal.
Source: Trane, What Is a Heat Pump Defrost Cycle?, checked 2026-10-22.
Normal
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A cloud of steam from the outdoor unit
That's the ice melting off during defrost. Trane calls it perfectly normal.
Source: Trane, What Is a Heat Pump Defrost Cycle?, checked 2026-10-22.
Normal
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The fan stops, the compressor sounds different
During defrost the outdoor fan may stop, and the compressor can get louder or quieter.
Source: Trane, What Is a Heat Pump Defrost Cycle?, checked 2026-10-22.
Normal
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AUX or Aux Heat on the thermostat
That's the backup electric heat keeping the house warm while the heat pump defrosts.
Source: Trane, What Is a Heat Pump Defrost Cycle?, checked 2026-10-22.
Normal
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Cool air from the vents for a few minutes
In defrost it can blow cool air. Heating resumes when the cycle ends.
Source: Trane, Troubleshooting Tips for a Heat Pump Not Heating, checked 2026-10-22.
Normal
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Ice that stays, or keeps growing
Give it a few hours to clear itself. If it doesn't, Trane points to low refrigerant, a faulty defrost control or a sensor.
Source: Trane, Heat Pump Troubleshooting, checked 2026-10-22.
Call us
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Defrosting constantly, for longer each time
Cycles that keep coming and lasting longer, with ice spreading over the unit, are a call for service.
Source: Trane, What Is a Heat Pump Defrost Cycle?, checked 2026-10-22.
Call us
Tell us what it's doing. We'll take it from there.
Call and describe the problem: the noise, the smell, the room that never warms up. Or book a visit online.
