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161036-YIM-E-1008
Defrost Operation
The demand defrost control implements a temperature
differential ("delta-T") demand defrost algorithm. The heat pump
is allowed to operate in the heating mode until the combination
of outdoor ambient and outdoor coil temperatures indicate that
defrosting is necessary (See Figure 13). When coil temperature
is below the initiate point for the ambient temperature
continuously for 4-1/2 minutes, the heat pump is put into a
defrost cycle. This 4-1/2 minute timer eliminates unnecessary
defrost cycles caused by refrigeration surges such as those that
occur at the start of a heating cycle.
A timed inhibit feature prevents the system from responding to
a call for defrost less than 20 minutes after the initiation of the
previous defrost. After the 20 minute inhibit time has expired,
temperature conditions must call for defrost continuously for 4-
1/2 minutes before a defrost cycle is initiated. A temperature
Figure 13: Defrost Operation Curves
Table 4: Demand Defrost Selection
Unit
SJ 024, 030, 042
SJ 036, 048, 060
12
REGION C
TERMINATE CURVE
Defrost Terminated
REGION E
No call for defrost
Time Defrost (6 hour) prevented
INHIBIT CURVE
REGION B
No call for Defrost
Time Defrost (6 hour) allowed
AMBIENT TEMPERATURE
Pin Position
2
3
inhibit feature prohibits defrost if the coil temperature is above
40°F.
A forced-defrost feature puts the system into a defrost period
every 6 hours and 4 minutes to recirculate lubricants, unless the
coil temperature is above 40°F. All defrost timing occurs only
while the compressor is on.
During the defrost mode, the defrost control will provide a 24
volt signal from terminal "W1/66" to the fan control terminal
"W1". This signal will energize electric heat stage 1, if the unit is
so equipped.
For trouble shooting purposes, the defrost cycle can be
manually initiated by shorting the "TEST" pins together for 5
seconds. Defrost will terminate normally during the "TEST"
mode.
REGION D
Defrost prevented from starting
REGION A
Call for Defrost
1
2
3
4
DEMAND
X/L
DEFROST
SELECTION
R
JUMPER
(JUMPER
C
IS SHOWN
Y
IN POSITION
#1)
O
W
W1/66
REV
VALVE
Figure 14: Demand Defrost "Curve" Selection Jumper
TEST
PRESSURE
M
SWITCH
Johnson Controls Unitary Products
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