Carrier Ultra Operator's Manual page 20

The defrost mode may be initiated in three different ways if the
evaporator coil is below 40°F (4.4°C):
1. Defrost is initiated automatically at preset intervals by the
defrost timer in the microprocessor.
2. Defrost is initiated by the defrost air switch.
3. The defrost mode may be manually initiated by pressing the
Manual Defrost Key.
The defrost mode terminates when the evaporator temperature is
higher than 55°F (12.8°C). Should the defrost cycle not complete
within 45 minutes, the defrost cycle is terminated.
After the 45 minute termination, the controller will wait 1.5 hours
before attempting another defrost cycle. Pressing the manual
defrost key will override this mode and start a defrost cycle.
If a shutdown alarm occurs, defrost will be terminated.
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