Short answer
Light frost on a heat pump's outdoor coil in winter is normal, and the unit clears it with periodic defrost cycles. A unit encased in solid ice, or ice that never clears, points to a defrost control problem, low refrigerant, a failed fan or water dripping onto it from above.
Normal defrost
Every 30 to 90 minutes in frosty weather, a heat pump reverses briefly to warm the outdoor coil. You'll see steam and hear a whoosh. That's normal.
Signs of a problem
- The coil and top of the unit are solid ice.
- The fan is blocked by ice.
- Ice builds after freezing rain and never clears.
- Backup heat runs constantly.
What you can do
Make sure the unit sits above snow level, clear snow away from the base, and check that a gutter isn't dripping on it. Don't chip at the ice. Warm water poured gently over the coil can clear it after an ice storm.
When to call a technician
- The unit stays encased in ice for more than a few hours.
- Heating output drops and emergency heat runs constantly.
What the technician does: see heat pump repair.
Common questions
Should I switch to emergency heat?
Only temporarily while you wait for service. Emergency heat is expensive to run.
Editorial note: this guide is general information for Kansas City-area homeowners, not a diagnosis. Conditions vary by home, so a licensed HVAC technician should confirm the cause before any repair.