Lennox 14HPX-018 Installation Instructions Manual page 20

OPERATIONAL DESCRIPTION
The defrost control board has three basic operational
modes: normal, calibration, and defrost.
Normal Mode--The demand defrost board monitors the
O line, to determine the system operating mode (heat/
cool), outdoor ambient temperature, coil temperature (out-
door coil) and compressor run time to determine when a
defrost cycle is required.
Calibration Mode--The board is considered uncalibrated
when power is applied to the board, after cool mode opera-
tion, or if the coil temperature exceeds the termination tem-
perature when it is in heat mode,
Calibration of the board 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,
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 board initi-
ates 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.
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 6 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 de-
mand 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 be-
low 35°F (2°C), the board logs the compressor run time. If
the board is not calibrated, a defrost cycle will be initiated
after 34 minutes of heating mode compressor run time.
The control will attempt to self-calibrate after this (and all
other) defrost cycle(s).
Calibration success depends on stable system tempera-
tures during the 20-minute calibration period. If the board
fails to calibrate, another defrost cycle will be initiated after
90 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 differ-
ence 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 board 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 be-
ing applied to the board, 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 ig-
nore 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 board 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).
Board timings are reduced, the low-pressure switch is ig-
nored and the board 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 to flow chart (figure 22)
for "TEST" operation.
Note: The Y1 input must be active (ON) and the "0" room
thermostat terminal into board must be inactive.
DEFROST BOARD DIAGNOSTICS
See table 14 to determine defrost board operational condi-
tions and to diagnose cause and solution to problems.
505243 04/08
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