876572-BIM-C-0213
Johnson Controls Unitary Products
15
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
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.
Figure 13: Defrost Operation Curves
REGION C
TERMINATE CURVE
INHIBIT CURVE
INITIA
TE CURVE
REGION A
REGION B
REGION D
REGION E
AMBIENT TEMPERATURE
Defrost prevented from starting
No call for defrost
Time Defrost (6 hour) prevented
No call for Defrost
Time Defrost (6 hour) allowed
Call for Defrost
Defrost Terminated
OUTDOOR (COIL)
TEMPERA
TURE
Table 4:
Demand Defrost Selection
Unit
Pin Position
UB 024, 030, 042
2
UB 036, 048, 060
2
TEST
T
N
E I
B
M
A
G
B
M
A
G
L I
O
C
L I
O
C
1
2
3
4
D
N
O
C
N
A
F
E
G
A
T
L
O
V
H
G I
H
PRESSURE
M
SWITCH
REV
VALVE
X/L
R
C
Y
O
W
W2/66
L /
X
R
C
Y
O
W
6
6 /
1
W
DEMAND
DEFROST
SELECTION
JUMPER
(JUMPER
IS SHOWN
IN POSITION
#2)