Fisher PTTW-10-EC Series Engineering Manual page 31

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THROUGH THE WALL UNIT
If OPT sensor fails, the unit will select automatic defrost function.
Automatic defrost function start condition:
Heating mode, and compressor continuously working for 25 minutes, if IPT-RT≤18℃, the unit
will go into defrost function.
Automatic defrost function stop condition:
When the compressor has worked for 12 minutes, unit will stop defrost function.