Lennox XP16 Series Unit Information page 39

IMPORTANT − The demand defrost control board will allow a
greater accumulation of frost and will initiate fewer defrost
cycles than a time/temperature defrost system.
Time If six hours of heating mode compressor run time has
elapsed since the last defrost cycle while the coil
temperature remains below 35°F (2°C), the demand defrost
control will initiate a defrost cycle.
Actuation When the reversing valve is de−energized, the
Y1 circuit is energized, and the coil temperature is below
35°F (2°C), the board logs the compressor run time. If the
board is not calibrated, a defrost cycle will be initiated after
30 minutes of heating mode compressor run time. The
control will attempt to self−calibrate after this (and all other)
defrost cycle(s).
XP16
Calibration success
temperatures during the 20−minute calibration period. If the
board fails to calibrate, another defrost cycle will be initiated
after 45 minutes of heating mode compressor run time.
Once the defrost board is calibrated, it initiates a demand
defrost cycle when the difference between the clear coil and
frosted coil temperatures exceeds the maximum difference
allowed by the control or after six hours of heating mode
compressor run time has been logged since the last defrost
cycle.
Termination The defrost cycle ends when the coil
temperature exceeds the termination temperature or after
14 minutes of defrost operation. If the defrost is terminated
by the 14−minute timer, another defrost cycle will be initiated
after 30 minutes of run time.
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depends on stable system