5156631-UIM-G-0417
Johnson Controls Unitary Products
25
Defrost Mode
The defrost mode is equivalent to the cooling mode except that the out-
door fan motor is de-energized.
If the call for heating is removed from the control during the defrost
cycle, the control terminates the defrost cycle. The control also stops
the “Defrost Cycle Timer,” but the control does not reset the “Defrost
Cycle Timer.” If conditions requiring defrost remain (coil temperature is
below 35°
F) and the control receives another call for compressor heat-
ing and then the call for heating is removed, the system restarts the
defrost cycle and the “Defrost Cycle Timer.”
The control remembers in what stage and in what part of the control
operation it is before initiating a defrost. The control runs 4
th
Stage com-
pressor during defrost operation and during calibration.
The control does the following to initiate a defrost cycle.
•
Communicate proper speed of compressor (High Stage – 4
th
Stage)
•
The Low Pressure Switch (LPS) is ignored during defrost.
•
Begin the “Defrost Cycle Timer”
•
De-energize the outdoor fan
•
Energize the reversing valve
•
Energize “Y2 OUT” for high ID airflow (if not already active)
•
Energize “W1 OUT”
for auxiliary heat and “W2 OUT” 15 minutes
following “W1 OUT”
being energized.
Defrost Termination
The control terminates the defrost cycle immediately after the coil tem-
perature goes above the selected termination temperature 50°
F, 60°
F,
70°
F, or 80°
F for 2 seconds. This selection is made per a shunt jumper
on the control board. If the terminate temperature selection shunt
jumper is not installed or more than one setting recognized, 50°
F
becomes the default termination temperature. The coil temperature
used to terminate the defrost cycle may be filtered to improve the per-
formance of the system.
Once a defrost mode is initiated, an internal timer (Defrost Cycle Timer)
counts the time that the defrost mode is engaged and that the compres-
sor is energized. After 12 minutes of operation in the defrost mode, the
defrost sequence terminates immediately and resets the internal tim-
ings regardless of the state of the coil sensor temperature.
Once a defrost mode is initiated, an internal timer (Coil T greater than
35° F Timer) counts the time that the coil temperature Coil T is above
35°
F. The control is able to indicate at the end of a defrost if the Coil T
is or is not more than 35°
F for 4 minutes. If the control determines that
it was above 35°
F for longer than 4 minutes and the control terminates
the defrost based on the Defrost Cycle Timer, the control accepts that
the coil is frost free, and it remains or enters the Demand Defrost Mode
and calibrates for the next defrost. If the control determines that it is not
above 35°
F for more than 4 minutes and the control terminates the
defrost based on the Defrost Cycle Timer, the control accepts that the
coil is NOT frost free, and it remains or enters the Time/Temperature
Defrost Mode.
The control immediately terminates a forced defrost cycle once forced
by shorting the TEST input when the “O” input is energized. This allows
the run test equipment to reduce the time required for production test-
ing. The control does not terminate a normal defrost cycle if it receives
an “O” signal during the defrost cycle.
The Low Pressure Switch is ignored during defrost and for 120 seconds
following termination of a Defrost.
The control establishes a new Dry Coil Delta T following termination of
this defrost cycle. The compressor run time resets when the defrost
cycle is complete.
Before returning to the previous compressor speed and to the same
place in the system starting operation when exiting defrost, the control
waits until after calibration and the next defrost is completed. The con-
trol does NOT de-energize the “Y2 OUT” for the high ID airflow until
after the calibration is completed
(unless required by the presents of a
heating call).
FIGURE 16:
Defrost Operation Curves (Variables - Defrost Active Variable)
A
0510-001
-40
(-40)
-20
(-29)
0
(-18)
20
(-7)
40
(4)
55
(13)
5(-15)
10(-12)
15(-9)
20(-7)
25(-4)
30(-1)
35(2)
4(-16)
9(-13)
11(12)
15(-9)
21(-6)
24(-4)
=
DE
F
ROST ACT
I
VE
VAR
I
A
BL
E
DE
L
TA T
° F(
C
)
CO
IL
T
° F (
C
)
Y
E
LL
O
W = F
ROST
ING
RE
GI
O
N
RED
=
DE
F
ROST RE
GI
O
N
G
REE
N = N
ORMA
L
HEAT
ING
OPERAT
I
O
N
35(2)
