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507640-05
Issue 2245
performance testing. The feature is deactivated by
removing the jumper located on the compressor delay pins
on the control board mounted inside the unit control box.
This feature is optional for the homeowner, but may impact
testing performance.
Defrost Operation
The defrost control system has three basic operational
modes: normal, calibration, and defrost.
• Normal Mode—
The demand defrost system 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 operation, or if the coil temperature
exceeds the termination temperature when it is in heat
mode.
Calibration of the board occurs after a defrost cycle
to ensure 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
detailed description of the defrost system operation.
Defrost Cycles
The control board initiates a defrost cycle based on either
frost detection or time.
• Frost Detection
—If the compressor runs longer than
30 minutes and the actual difference between the
clear coil and frosted coil temperatures exceeds the
maximum difference allowed by the control, a defrost
cycle will be initiated.
IMPORTANT
-
The 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 control
board 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).
Calibration success depends on stable system
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 control 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 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 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.
5-Strike Lockout Feature
The internal control logic of the board counts the pressure
switch trips only while the Y1 (Input) line is active. If a
pressure switch opens and closes four times during a Y1
(Input), the control logic will reset the pressure switch trip
counter to zero at the end of the Y1 (Input). If the pressure
switch opens for a fifth time during the current Y1 (Input),
the control will enter a lockout condition.
The 5-strike pressure switch lockout condition can be reset
by cycling OFF the 24-volt power to the control board or by
shorting the TEST pins between 1 and 2 seconds. All timer
functions (run times) will also be reset.
If a pressure switch opens while the Y1 Out line is engaged,
a 5-minute short cycle will occur after the switch closes.
Diagnostic LEDs
The defrost board uses two LEDs for diagnostics. The
LEDs flash a specific sequence according to the condition
as shown in Table 4.
