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XPG20 SERIES
Defrost System
DEFROST SYSTEM OVERVIEW
The defrost control board (DCB) measures differential
temperatures to detect when the system is performing
poorly because of ice build−up on the outdoor coil. The
controller self−calibrates when the defrost system starts
and after each system defrost cycle. The defrost control
board components are illustrated on the previous page.
The DCB monitors ambient temperature, outdoor coil
temperature, and total run time to determine when a
defrost cycle is required. The coil temperature probe has a
spring clip to allow mounting to the outside coil tubing. The
location of the coil sensor is important for proper defrost
operation.
NOTE The DCB accurately measures the performance
of the system as frost accumulates on the outdoor coil.
This typically will translate into longer running time
between defrost cycles as more frost accumulates on the
outdoor coil before the board initiates defrost cycles.
DCB PRESSURE SWITCH CONNECTIONS
The unit’s reset pressure switches (LO PS − S87 and HI PS
− S4) are factory−wired into the DCB on the LO−PS and
HI−PS terminals.
1.
Low Pressure Switch (LO−PS)
When the low
pressure switch trips, the DCB will cycle off the
compressor, and the strike counter in the DCB will
count one strike. The low pressure switch is ignored
under the following conditions:
S
During the defrost cycle and 90 seconds after the
termination of defrost
S
When the average ambient sensor temperature is
below 15° F (−9°C)
S
For 90 seconds following the start up of the
compressor
S
During TEST mode
2.
High Pressure Switch (HI−PS)
When the high
pressure switch trips, the DCB will cycle off the
compressor, and the strike counter in the board will
count one strike.
DCB PRESSURE SWITCH EVENT SETTINGS
The following pressures are the auto reset event values for
low and high pressure thresholds:
S
High Pressure
(auto reset) − trip at 590 psig; reset at
418.
S
Low Pressure
(auto reset) − trip at 25 psig; reset at 40.
FIVE−STRIKE LOCKOUT SAFETY FUNCTION
The five−strike lockout safety function is designed to
protect the unit’s compressor from damage. The DCB
looks for 24VAC on its Y1 terminal. When the Y1 input
detects 24VAC, the DCB internal control logic will perform
the following:
S
Count any HI−PS and LO−PS pressure switch trips
(open and close).
NOTE Individual HI−PS and LO−PS trips are totaled by
the DCB.
S
Up to four pressure switch trips are allowed in a single
thermostat demand without locking out the DCB. If the
thermostat demand is satisfied before a fifth pressure
switch trip, the control will reset the five−strike counter
to zero.
S
If either pressure switch opens for a fifth time during
a single Y1 demand, the DCB will enter a lockout
condition.
NOTE If 5−strikes occur, within a single room thermostat
demand, the board will lockout the unit.
The system will require servicing to determine the cause of
the pressure switch condition. Once the condition has
been rectified, reset the DCB
.
DCB MANUAL RESET
The defrost control board’s 24VAC power
R
terminal must be cycled OFF, or the TEST
pins must be jumpered for one second but
less than two seconds to reset the DCB.
DCB OPERATIONAL MODES
The defrost control board has three basic
operational modes: normal, calibration,
and defrost.
1.
Normal Mode
The demand DCB
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.
2.
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.
3.
Defrost Mode
The following paragraphs provide a
detailed description of the defrost system operation.
DEFROST CYCLE OPERATION
1.
Defrost Cycles
The demand defrost control board
initiates a defrost cycle based on either frost detection
or time.
S
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 demand defrost control board will
allow a greater accumulation of frost and will initiate
fewer defrost cycles than a time/temperature defrost
system.
S
Time
If six 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 demand defrost control will initiate a
defrost cycle.
