835961-UIM-D-0213
Johnson Controls Unitary Products
11
Demand Defrost (13 Seer 1.5 ton, 14.5 Seer 1.5-4 Ton)
The control maintains proper airflow through the outdoor coil during
heating operation by melting frost and ice that may form on the coil.
Frost may accumulate unevenly in different sections of the coil because
of the arrangement of the refrigeration circuit within the coil. The control
may initiate a defrost cycle even when the coil is not completely cov-
ered with frost. This is normal operation.
The control regulates the defrost operation of the heat pump based on
accumulated compressor run time, outdoor coil temperature, and out-
door ambient temperature. The control will cause the unit to operate in
the normal heating mode until it determines that a defrost cycle is
needed.
All defrost timings are based on accumulated compressor run time.
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 W1/66 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 liquid line 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 in Figure 13. Table 4 shows the jumper
position that is appropriate for each heat pump model. Jumper position
4 is not used and the control will not allow the compressor to operate
when the jumper is in this position.
Defrost Curve Selection
The factory will place the defrost curve selection jumper in the P posi-
tion or in a numbered position appropriate for the specific heat pump
model. You should not have to change the defrost curve selection
jumper during initial installation.
If the jumper is inadvertently moved, it should be placed in the appropri-
ate numbered location based on the model number and Table 4. The
control will also not energize the compressor if the defrost curve selec-
tion jumper is in a numbered position that is not described in Table 4 or
if the defrost curve selection jumper is missing. The control will display
the proper fault code when a defrost curve jumper error is present. The
control will display the active defrost curve using the X/L terminal when
the heat pump is operating in a defrost cycle that has been forced using
the TEST inputs.
For instance, the X/L output will be energized with two flashes when
defrost curve 2 is active. 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 liquid line temperature
is below the initiate point for the measured ambient temperature (See
Figure 13) continuously for 4-1/2 minutes. This delay eliminates unnec-
essary 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 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 six seconds with a Y input energized and the
pressure switch input is closed, the ASCD will be bypassed and the
compressor and the W1/66 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 liquid line temperature and outdoor ambient tem-
perature inputs. The coil does not have to be cold and the outdoor tem-
perature 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.
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 80°F or after eight minutes of defrost opera-
tion.
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 W1/66 terminal.
•
Reset and restart the 40-minute defrost inhibit timer.
Compressor Delay
When Defrost Jumper Position #2 is selected the compressor is shut
down for 30 seconds entering and exiting defrost mode. This delay is
present in normal operation but is not present when the TEST pins are
shorted to force a defrost cycle. Position #1 is recommended for recip-
rocating compressors, and Position #2 is recommended for scroll com-
pressors.
Jumper settings 1-3 are different defrost curve settings. 4 & P will not work in application.
The defrost thermostat delay will make the coil temperature about 75-
80° F. Please note that the timer will stop the circuit when R to Y is
disconnected.
NOTICE
TABLE 4:
Defrost Initiate Curves
Defrost Curve Selection
Jumper Position
1
2
3
4
All Other Models
YHJR, THJR,THRD, GHRD
YHJD, YHJF, THJD, THJF,
CHJD, CHJF, LHJD, LHJF
THGD, GHGD
None
None
