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Defrost Thermostat
The defrost thermostat is located on the liquid line between the
check/expansion valve and the distributor. When defrost
thermostat senses 42ºF or cooler, the thermostat contacts close
and send a signal to the defrost control board to start the defrost
timing. It also terminates defrost when the liquid line warms up to
70ºF.
Defrost Control
The defrost control board includes the combined functions of the
time/temperature defrost control, defrost relay, diagnostic LEDs
and terminal strip for field wiring connections. See "Defrost
Control Board" illustration.
The control provides automatic switching from normal heating
operation to defrost mode and back. During compressor cycle
(call for defrost), the control accumulates compressor run times
at 30, 60 or 90 minute field-adjustable intervals. If the defrost
thermostat is closed when the selected compressor run time
interval ends, the defrost relay is energized and defrost begins.
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. See "Defrost Control Board" illustration. The defrost
timing jumper is factory-installed to provide a 60-minute defrost
interval. 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.
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 re-applied.
Pressure Switch Circuit
The defrost control incorporates a pressure switch circuit that
allows the application of an optional high pressure switch. See
"Defrost Control Board" illustration. During a demand cycle, the
defrost control will lock out the unit if the optional high pressure
switch opens. The diagnostic LEDs will display a pattern for an
open high pressure switch. See "Diagnostic LEDs." The unit will
remain locked out until the switch resets or is reset.
Remove the factory-installed jumper before connecting the
optional high pressure switch to the control board.
NOTE: If not using a pressure switch, the factory-installed jumper
wire must be connected.
8
Adjusting Defrost System
Defrost Control Board
Optional high pressure switch safety circuit connections
NOTE: To add the pressure switch, remove the factory-installed
jumper.
P1
30
A
60
90
TEST
B
P5
C
D
O-OUT
LO-PS
E
DF
F
Y1-OUT
K3 Relay
HI-PS
G
A. Defrost interval timing pins
B. Test pins
C. Compressor delay pins
D. Reversing valve
E. Low pressure switch
Adjust Time Between Defrost Cycles
WARNING
Electrical Shock Hazard
Disconnect power before servicing.
Replace all parts and panels before operating.
Failure to do so can result in death or electrical shock.
1. Disconnect power.
2. Remove the control box cover.
FAN
C2
K1 Relay
DS1
H
DS2
U2
K2 Relay
U1
P2
W1
L
C
24V
I
L
R
C5
O
Y1
P6
TST PS DF C
R O Y1
F. Defrost thermostat
G. High pressure switch (optional)
H. Diagnostic LEDs
I. 24V terminal strip connections
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