JOHNSON CONTROLS
142
Form 150.63-NM9
Issue date: 5/20/2021
SECTION 8 – UNIT OPERATION
System lead/lag
(Discharge Air Temp Control Only)
Lead/lag between systems may be selected to help
equalize average run hours between systems on con-
densing units with two refrigerant systems. This may
be programmed under the OPTIONS key. Auto lead/
lag allows automatic lead/lag of the two systems based
on average run hours of the compressors in each sys-
tem. Manual lead/lag selects specifically the sequence
in which the microboard starts systems. Systems in
Suction Pressure Control act independently based on
the individual Suction Pressure Control setpoints.
Compressor lead/lag
The compressors within a system rotate starts in se-
quence 1, 2 or 1, 2, 3 with wraparound. The longest
OFF compressor in a system will start first, and the
longest-running compressor in a system will turn OFF
first. When unloading, the system with the most com-
pressors on unloads first. The lag system will shut
down a compressor first when equal numbers of com-
pressors are operating in each system. The microboard
will not attempt to equalize run time of compressors in
a system.
Once the second system has started a compressor, the
microboard will attempt to equally load each system.
Once this occurs, loading will alternate between sys-
tems.
If Soft Start is enabled on European models with this
option, compressor lead/lag will function as outlined in
Option 12 under the OPTIONS key.
Anti-recycle timer
The programmable anti-recycle timer ensures that sys-
tems do not cycle. This timer is programmable under
the PROGRAM key between 300 and 600 seconds.
Whenever possible, to reduce cycling and motor heat-
ing, the anti-recycle timer should be adjusted to 600
seconds. The programmable anti-recycle timer starts
the timer when the first compressor in a system starts.
The timer begins to count down. If all of the compres-
sors in a circuit cycle OFF, a compressor within the cir-
cuit will not be permitted to start until the anti-recycle
timer has timed out. If the lead system has run for less
than five minutes, three times in a row, the anti-recycle
timer will be extended to 10 minutes.
Anti-coincidence timer
This timer is not present on single-system units. Two
timing controls are present in software to ensure com-
pressors within a circuit or between systems, do not start
simultaneously. The anti-coincidence timer ensures
there is at least a one minute delay between system starts
on two circuit systems. This timer is NOT programma-
ble. The load timers further ensure that there is a mini-
mum time between compressor starts within a system.
Evaporator blower control
The evaporator blower start contacts (TB1 - terminals 23
- 24) are energized when any of the following conditions
are true:
1. Any compressor is running.
2. Daily Schedule is not programmed OFF and
Unit Switch is ON.
The contacts will not close if the micropanel has been
powered up for less than 30 seconds or if the contacts
have been closed in the last 30 seconds to prevent mo-
tor overheating. These contacts can be used to start the
evaporator blower, and for all practical purposes, will
be running in a “constant fan” mode. However, if the
blower is desired to cycle with the compressors, then
cycling of the evaporator blower can be achieved by
using the “Run Contacts” located at or TB1 - terminals
27 to 28 for system two (if applicable).
Evaporator heater control
The evaporator heater is controlled by ambient air tem-
perature. When the ambient temperature drops below
40°F (4.4°C) the heater is turned ON. When the tem-
perature rises above 45°F (7.2°C) the heater is turned
OFF. An under voltage condition will keep the heater
off until full voltage is restored to the system.
Pumpdown control
Each system has a pump-down feature upon shut-
off. Manual pumpdown from the keypad is possible
by turning off the respective system’s switch under
the OPTIONS key. On a non-safety, non-unit switch
shutdown, all compressors but one in the system will
be shut OFF. The LLSV will also be turned OFF. The
final compressor will be allowed to run until the suc-
tion pressure falls below the cutout, or for 180 seconds,
whichever comes first.
Summary of Contents for YCUL0020
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