5121495-UIM-E-0716
Johnson Controls Unitary Products
17
Operation
The defrost mode is equivalent to the cooling mode except that the out-
door fan motor is de-energized. The control shall do the following to ini-
tiate a defrost cycle.
•
De-energize the outdoor fan.
•
Energize the reversing valve.
•
Energize the auxiliary heat output through the Wout terminal.
•
Begin the maximum defrost cycle length timer.
If the call for heating (Y) is removed from the control during the defrost
cycle, it will terminate the defrost cycle and de-energize the compres-
sor. The control will also stop the defrost cycle length timer but not reset
it. When the control receives another call for heating, it will restart the
defrost cycle and the timer at the point at which the call for heating was
removed. This will happen only if the coil sensor temperature conditions
allow defrost to occur.
Defrost Curves
The control uses a set of defrost curve parameters that are selected
using the defrost curve selection jumper. The location of the defrost
curve selection jumper is shown on the Tabular Data sheet for each
heat pump model.
Defrost Curve Selection
The second page of the tabular data sheet indicates the proper jumper
setting for the specific heat pump model.
The control only reads the jumper input when the Y and W thermostat
inputs are de-energized. If a jumper position is changed while either of
these inputs is energized, the control will not act upon the jumper
changes until the thermostat calls are de-energized or power (24 VAC)
to the control is cycled.
Defrost Cycle Initiation
The control will allow the heat pump to operate in the heating mode until
the combination of outdoor ambient and outdoor coil temperatures indi-
cate that a defrost cycle is necessary.
The control will initiate a defrost cycle when the coil temperature is
below the initiate point for the measured ambient temperature (See Fig-
ure 14) continuously for 4-1/2 minutes. This delay eliminates unneces-
sary defrost cycles caused by refrigeration surges such as those that
occur at the start of a heating cycle.
The control will initiate a defrost cycle every 6 hours (accumulated com-
pressor run time) to recirculate refrigerant lubricants. This forced
defrost timer will be reset and restarted following the completion or ter-
mination of a defrost cycle.
The control shall initiate a defrost cycle when the Defrost Inhibit Time
Limit has elapsed if the previous defrost cycle was terminated based on
the Maximum Defrost Cycle Time. This shall occur regardless of the liq-
uid line (coil) temperature reading. The coil does not have to be cold for
the unit to be forced into defrost. Once the defrost cycle begins the con-
trol shall follow the normal defrost cycle routine.
The control will also initiate a defrost cycle when the TEST terminals
are shorted. This feature allows an installer or service technician to start
a defrost cycle immediately as required. When the TEST terminals are
shorted for more than 5 seconds with a Y input energized and the pres-
sure switch input is closed, the ASCD will be bypassed, the reversing
valve will be energized, the ODF will be de-energized and the compres-
sor and the Wout terminal to auxiliary heat will be energized.
When the TEST inputs are used to force a defrost cycle, the control will
ignore the state of the coil temperature and outdoor ambient tempera-
ture inputs. The coil does not have to be cold and the outdoor tempera-
ture does not have to be within a certain range for the heat pump to be
forced into a defrost cycle. After the TEST input jumper is removed, the
defrost mode will be terminated as normal. The defrost cycle length
timer will not be started until the TEST input is removed. If the TEST ter-
minals remain shorted, the control will keep the unit in defrost mode.
Defrost Inhibition
The control will not initiate a defrost cycle if the liquid line temperature is
above 40°F unless the defrost cycle is forced using the TEST input, or
the previous defrost exited on maximum time (12 minutes).
The control will also prevent a defrost cycle from being initiated too
soon after the initiation of the previous defrost cycle. When power is
applied to the control and after the completion or termination of each
defrost cycle, the control will start a 40-minute timer. When this timer
expires, the control will allow another defrost cycle when needed. The
timer is based on accumulated compressor run time.
Defrost Termination
The control will terminate the defrost cycle immediately after the liquid
line temperature reaches the terminate temperature or after 12 minutes
of defrost operation. See Figure 14.
The control will do the following to terminate a defrost cycle:
•
Energize the outdoor fan.
•
De-energize the reversing valve.
•
De-energize the auxiliary heat output through the Wout terminal.
•
Reset and restart the 40-minute defrost inhibit timer.
FIGURE 14:
Defrost Operation Curves
RE
G
ION C
TER
M
INATE CURVE
IN
H
I
B
IT CURVE
INITIA
TE
CUR
VE
RE
G
ION A
RE
G
ION
B
RE
G
ION D
RE
G
ION E
A
MB
IENT TE
M
PERATURE
LIQUID LINE
(
COIL
)
TE
M
PERA
TURE
D
e
f
rost
prevente
d f
rom
starting
N
o
ca
ll f
or
d
e
f
rost
T
ime
D
e
f
rost
(6
hour)
prevente
d
N
o
ca
ll f
or
D
e
f
rost
T
ime
D
e
f
rost
(6
hour)
a
ll
owe
d
C
a
ll f
or
D
e
f
rost
D
e
f
rost
T
erminate
d
A
0144-001
