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3. At the end of the defrost cycle, when the unit goes
back to heating mode, the low pressure switch is
checked to see if it has reset. If so, the strikeout is not
counted. This prevents lockout during extreme winter
conditions.
Defrost Control Timing Pins
Each timing pin selection provides a different accumulated
compressor run time period during one thermostat run
cycle. This time period must occur before a defrost cycle
is initiated. The defrost interval can be adjusted to 30
(T1), 60 (T2), or 90 (T3) minutes.
It is intended that this
product should be set at the 60-minute time interval at
initial installation.
If the timing selector jumper is not in
place, the control defaults to a 90-minute defrost interval.
The maximum defrost period is 14 minutes and cannot be
adjusted.
For geographic areas that experience low temperature
and high humidity conditions (below 35°F and above
80% RH), the defrost timer pin must be field set at
installation to a 60 or 30 minute defrost interval to
ensure reliable system operation while in heating mode.
NOTE:
A test option is provided for troubleshooting. The test mode
may be started any time the unit is in the heating mode and
the defrost thermostat is closed or jumpered. If the jumper
is in the TEST position at power up, the control will ignore
the test pins. When the jumper is placed across the TEST
pins for 2 seconds, the control will enter the defrost mode.
If the jumper is removed before an additional 5-second
period has elapsed (7 seconds total), the unit will remain
in defrost mode until the defrost thermostat opens or 14
minutes have passed. If the jumper is not removed until
after the additional 5-second period has elapsed, the
defrost will terminate and the test option will not function
again until the jumper is removed and reapplied.
Compressor Delay (Quiet Shift)
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. The compressor will be
cycled off for 30 seconds going in and out of the defrost
mode when the compressor delay jumper is removed.
NOTE
:
The 30-second “off” cycle is not functional when
jumpering the TEST pins.
Time Delay
The defrost control includes a compressor timer, which
ensures the compressor is off for a minimum amount of
time between operating cycles.
The timed-off delay is 5 minutes long. The delay helps
to protect the compressor from short cycling in case the
power to the unit is interrupted or a pressure switch opens.
The delay is bypassed by placing the timer select jumper
across the TEST pins for 0.5 seconds.
Pressure Switch Circuit
High and low pressure switches are connected to the
defrost control board on heat pump models. Air conditioning
models have a high pressure switch installed in line with
compressor contactor coil (see Figure 6).
During a single demand cycle, the defrost control will
lock out the unit after the fifth time that the circuit is
interrupted by any pressure switch wired to the control
board. In addition, the diagnostic LEDs will indicate a
locked-out pressure switch after the fifth occurrence
of an open pressure switch (see Table 3).
The unit will remain locked out until power to the board
is interrupted, then re-established, or until the jumper
is applied to the TEST pins for 0.5 seconds.
NOTE
:
The defrost control board ignores input from the
low pressure switch terminals as follows:
• During the TEST mode
• During the defrost cycle
• During the 90-second start-up period
•
For the first 90 seconds each time the reversing valve
switches heat/cool modes
If the TEST pins are jumpered and the 5-minute delay
is being bypassed, the LO PS terminal signal is not
ignored during the 90-second start-up period.
Diagnostic LEDs
The defrost board uses two LEDs for diagnostics. The
LEDs flash a specific sequence according to the condition
as shown in Table 3.
Defrost Board Diagnostic LEDs
Green LED
(DS2)
Red LED
(DS1)
Condition
OFF
OFF
No Power to Control
Simultaneous slow FLASH
Normal Operation /
Power to Control
Alternating Slow FLASH
5-min Anti-Short-Cycle Delay
ON
Slow FLASH
Low Pressure Switch Ignored
(Low Ambient)
Fault & Lockout Codes
OFF
Slow FLASH
Low Pressure Switch Fault
OFF
ON
Low Pressure Switch Lockout
Slow FLASH
OFF
High Pressure Switch Fault
ON
OFF
High Pressure Switch Lockout
Table 6. Defrost Control (CMC1) Diagnostic LEDs
