170761-UIM-B-0406
12
Unitary Products Group
DEFROST OPERATION
General
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 crankcase heater
•
Energize the reversing valve
•
Energize the auxiliary heat outputs based on the system configu-
ration
•
Energize Y2Out terminal if not already energized
•
Begin the maximum defrost cycle length timer
If the call for heating (Y1) 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 Figures 10 and 13. Table 9 shows
the jumper position that is appropriate for each heat pump model.
Jumper positions 5, and 6 are not used and the control will not allow the
compressor to operate when the jumper is in any of these positions.
Defrost Curve Selection
The factory activates the correct defrost curve during production. They
will place the defrost curve selection jumper in the PRGM position or in
a numbered position appropriate for the specific heat pump model. You
should not have to change the defrost curve selection jumper during ini-
tial installation.
If the jumper is inadvertently moved, it should be placed in the appropri-
ate numbered location based on the model number and Table 9. If the
factory has activated the curve using the PRGM position, the jumper
may also be returned to that position. If, however, the factory has not
activated the curve in the PRGM position and the jumper is placed in
the PRGM position, the control will not energize the compressor. 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 9 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. If the
jumper is missing, the control will behave as if the jumper was in the
PRGM position. If the jumper is placed in a numbered position, the
defrost curve selected by the jumper will override the defrost curve acti-
vated at the factory until the jumper is returned to the PRGM position.
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. It will also display the active defrost curve using
the X/L terminal when the operational mode is being displayed using
the LED’s. For instance, the X/L output will be energized with two
flashes when defrost curve 2 is active. The control will lock out the com-
pressor if the defrost curve selection jumper is not properly set.
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 11) 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 Y1 input energized and the
high-pressure switch closed, the ASCD will be bypassed and the com-
pressor will be energized. If an O signal is present, the control will not
initiate a defrost cycle. If the defrost cycle is initiated using the TEST
terminals, the control will bypass the normal auxiliary heat timings and
will energize the W1 Out and W2 Out terminals immediately when it
begins the defrost cycle.
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 40F unless the defrost cycle is forced using the TEST input.
The control will not initiate a defrost cycle when the outdoor ambient
temperature is below –25F or above 55F 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 80F or after eight minutes of defrost opera-
tion.
The control will also terminate a defrost cycle that has been forced
using the TEST input when the O input is energized. The control will not
terminate a normal defrost cycle when it receives an O input.
The control will do the following to terminate a defrost cycle.
•
Energize the outdoor fan
•
De-energize the crankcase heater
•
De-energize the reversing valve
•
De-energize the auxiliary heat outputs
•
Control the Y2 Out terminal based on operating conditions
•
Reset and restart the 40-minute defrost inhibit timer
