In January, the average low in Chapel Hill is 29 degrees and morning humidity in the Triangle averages 79%, according to NOAA. That combination frosts a heat pump coil faster than a dry cold snap does. A thin layer of frost on the outside unit, followed a few minutes later by a cloud of steam, is the system clearing itself.
How defrost works
In heat mode, the outdoor coil runs 10 to 20 degrees colder than the air around it. Moisture freezes on the fins whenever the temperature sits in the 20s or 30s, and sometimes in the low 40s. Most heat pumps check for frost every 30 to 90 minutes. When the control finds enough of it, the system reverses and sends warm refrigerant through the outdoor coil to melt the ice.
During a defrost the outdoor fan stops, the unit makes a loud whoosh when the valve switches, and steam rises off the coil. Trane puts a typical defrost at 3 to 5 minutes and Lennox at 5 to 15. The thermostat may show Aux heat while it runs, because the indoor unit turns on its backup heat so the vents keep blowing warm air.
When the ice is a problem
Carrier draws the line at a quarter inch. Frost thinner than that is normal. Ice thicker than a quarter inch, or a solid coat that is still there after a defrost cycle, means a part has failed. Lennox says to call a technician if the ice has not melted within 6 to 8 hours or keeps coming back. A unit that switches into defrost over and over while the ice grows points to the same kind of failure.
Check the gutter above the unit before you call. Water dripping from a gutter onto the top of a heat pump freezes into a sheet that defrost cannot clear, and fixing the gutter fixes the ice.
What you can do
Keep 2 feet clear around the unit on every side, including snow and fallen leaves. Replace the indoor filter every 1 to 3 months. Do not chip at the ice or pour hot water on the coil, since Lennox warns against both and the aluminum fins bend easily.
If the outside unit has stopped running or stays iced solid, switch the thermostat to Emergency heat until a technician looks at it. Emergency heat runs only the electric backup strips. Carrier rates a heat pump at up to 300% efficient, which means about three times as much heat per kilowatt-hour as the strips, so a week on emergency heat shows up on the next power bill. Switch back once the repair is done.
What usually fails
When ice will not clear, Carrier, Trane and Lennox point to the same list of causes: a failed defrost control board or sensor, an outdoor fan motor that stopped turning, low refrigerant from a leak, or a reversing valve that sticks.
| Repair | Typical price |
|---|---|
| Diagnostic visit | $89 to $150 |
| Defrost control board | $300 to $700 |
| Outdoor fan motor | $250 to $650 |
| Refrigerant leak search and recharge | $400 to $1,500 |
| Reversing valve | $1,200 to $2,500 |
If the ice is still there after a few hours, call (980) 245-1226 and we will send a licensed heat pump repair technician.