Carrier X4 7700 Operator's Manual page 30

Defrost may also be initiated automatically at preset intervals by the system
defrost timer or by the defrost air switch.
The defrost mode terminates when the defrost termination temperature
sensor (DTT) and the supply air temperature sensor (SAT) both rise higher
than 55°F (12.8°C). Should the defrost cycle not end after a maximum of 45
minutes, the defrost cycle is terminated automatically. "DEFROST NOT
COMPLETE" will be displayed in the MessageCenter.
If defrost terminates on the 45-minute termination timer, the system will wait
1.5 hours of compressor running time before attempting an automatic defrost
cycle. Pressing the manual defrost key will override this mode and start a
defrost cycle.
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