Lennox XP14 SERIES Unit Information page 6

NOTE − The 30 second off cycle is NOT functional when
jumpering the TEST pins.
Operational Description
The defrost control has three basic operational modes:
normal, calibration, and defrost.
Normal Mode The demand defrost control monitors the
O line, to determine the system operating mode (heat/cool),
outdoor ambient temperature, coil temperature (outdoor
coil) and compressor run time to determine when a defrost
cycle is required.
Calibration Mode The control is considered uncali-
brated when power is applied to the control, after cool mode
operation, or if the coil temperature exceeds the termina-
tion temperature when it is in heat mode.
Calibration of the control occurs after a defrost cycle to en-
sure that there is no ice on the coil. During calibration, the
temperature of both the coil and the ambient sensor are
measured to establish the temperature differential which is
required to allow a defrost cycle. See figure 8 for calibration
mode sequence.
Defrost Mode The following paragraphs provide a de-
tailed description of the defrost system operation.
Detailed Defrost System Operation
Defrost Cycles The demand defrost control initiates a
defrost cycle based on either frost detection or time.
Frost Detection If the compressor runs longer than 34
minutes and the actual difference between the clear
coil and frosted coil temperatures exceeds the maxi-
mum difference allowed by the control, a defrost cycle
will be initiated.
Time If 6 hours of heating mode compressor run time has
elapsed since the last defrost cycle while the coil tem-
perature remains below 35°F (2°C), the demand de-
frost 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 control logs the compressor run time. If the
control is not calibrated, a defrost cycle will be initiated after
34 minutes of heating mode compressor run time. The con-
trol will attempt to self−calibrate after this (and all other) de-
frost cycle(s).
Calibration success depends on stable system tempera-
tures during the 20−minute calibration period. If the control
fails to calibrate, another defrost cycle will be initiated after
45 minutes (90 minutes −1 to −4 boards) of heating mode
compressor run time. Once the defrost control is calibrated,
it initiates a demand defrost cycle when the difference be-
tween the clear coil and frosted coil temperatures exceeds
the maximum difference allowed by the control OR after 6
hours of heating mode compressor run time has been
logged since the last defrost cycle.
NOTE − If ambient or coil fault is detected, the control will not
execute the TEST" mode.
Termination The defrost cycle ends when the coil tem-
perature 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 34 minutes of run time.
Test Mode When Y1 is energized and 24V power is being
applied to the control, a test cycle can be initiated by placing
the termination temperature jumper across the Test" pins
for 2 to 5 seconds. If the jumper remains across the Test"
pins longer than 5 seconds, the control will ignore the test
pins and revert to normal operation. The jumper will initiate
one cycle per test.
Enter the TEST" mode by placing a shunt (jumper) across
the TEST" pins on the control after power−up. (The TEST"
pins are ignored and the test function is locked out if the
shunt is applied on the TEST" pins before power−up). Con-
trol timings are reduced, the low−pressure switch is ignored
and the control will clear any active lockout condition.
Each test pin shorting will result in one test event. For
each TEST" the shunt (jumper) must be removed for at
least 1 second and reapplied. Refer Defrost Control Pin Op-
erationto flow chart (figure 7) for TEST" operation.
Note: The Y1 input must be active (ON) and the O" room
thermostat terminal into board must be inactive.
Defrost Control Diagnostics
See table 2 to determine defrost control operational condi-
tions and to diagnose cause and solution to problems.
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