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Second−Stage Operation
If the board receives a call for
second−stage compressor operation Y2" in heating or
cooling mode and the first-stage compressor output is ac-
tive, the second-stage compressor solenoid output will be
energized.
If first-stage compressor output is active in heating mode
and the outdoor ambient temperature is below the selected
compressor lock−in temperature, the second-stage com-
pressor solenoid output will be energized without the Y2"
room thermostat input. If the jumper is not connected to one
of the temperature selection pins on P3 (40, 45, 50, 55°F),
the default lock−in temperature of 40°F (4.5°C) will be used.
The board de−energizes the second-stage compressor so-
lenoid output immediately when the Y2" signal is removed
or the outdoor ambient temperature is 5°F above the se-
lected compressor lock−in temperature, or the first-stage
compressor output is de−energized for any reason.
Defrost Temperature Termination Shunt (Jumper) Pins
The defrost board selections are: 50, 70, 90, and 100°F
(10, 21, 32 and 38°C). The shunt termination pin is factory
set at 50°F (10°C). If the temperature shunt is not installed,
the default termination temperature is 90°F (32°C).
Delay Mode
The defrost board has a field−selectable function to reduce
occasional sounds that may occur while the unit is cycling in
and out of the defrost mode. When a jumper is installed on
the DELAY pins, the compressor will be cycled off for 30
seconds going in and out of the defrost mode. Units are
shipped with jumper installed on DELAY pins.
NOTE − The 30 second off cycle is NOT functional when the
TEST pins are jumpered.
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 (outdoor
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 al-
low a greater accumulation of frost and will initiate fewer de-
frost 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
45 minutes (90 minutes −1 to −4 boards) of heating mode
compressor run time. Once the defrost board 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 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 plac-
ing 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 ini-
tiate 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 and
loss of charge detection fault is ignored 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 8)
for TEST" operation.
NOTE The Y1 input must be active (ON) and the O"
room thermostat terminal into board must be inactive.
Defrost Board Diagnostics
See table 2 to determine defrost board operational condi-
tions and to diagnose cause and solution to problems.
