157650-UIM-B-1105
Unitary Products Group
7
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. Table 3 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 activates the correct defrost curve during production. They
will place the defrost curve selection jumper in the P 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 3. If the
factory has activated the curve using the P position, the jumper may
also be returned to that position. If, however, the factory has not acti-
vated the curve in the P position and the jumper is placed in the P posi-
tion, the control will not energize the compressor. The control will also
not energize the compressor if the defrost curve selection jumper is in a
numbered position that is not described in Table 3 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 miss-
ing, the control will behave as if the jumper was in the P position. If the
jumper is placed in a numbered position, the defrost curve selected by
the jumper will override the defrost curve activated at the factory until
the jumper is returned to the P position. The control will display the
active defrost curve using the X/L terminal when the heat pump is oper-
ating 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 only reads the jumper input when the Y and W ther-
mostat 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 10) 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 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 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
TABLE 3: Defrost Initiate Curves
Defrost Curve Selection Jumper Position
1
2
3
4
13 Seer
All
None
None
None
