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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 2 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 system.
Manual lead/lag selects specifically the sequence in
which the micro starts systems. Systems in Suction
Pressure Control act independently based on the indi-
vidual 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 lon-
gest-running compressor in a system will turn off first.
When unloading, the system with the most compres-
sors on unloads first. The lag system will shut down a
compressor first when equal numbers of compressors
are operating in each system. The micro will not at-
tempt to equalize run time of compressors in a system.
Once the second system has started a compressor, the
micro will attempt to equally load each system. Once
this occurs, loading will alternate between systems.
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 assures that
systems do not cycle. This timer is programmable un-
der the PROGRAM key between 300 – 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 5 minutes, 3 times in a row, the anti-recycle timer
will be extended to 10 minutes.
128
ANTI-COINCIDENCE TIMER
This timer is not present on single-system units. Two
timing controls are present in software to assure com-
pressors within a circuit or between systems, do not
start simultaneously. The anti-coincidence timer assures
there is at least a one minute delay between system starts
on 2-circuit systems. This timer is NOT programmable.
The load timers further assure that there is a minimum
time between compressor starts within a system.
EVAPORATOR BLOWER CONTROL
The evaporator blower start contacts (XTBC2 - termi-
nals 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 XTBC2 - termi-
nals 25 to 26 for system 1, or XTBC2 - terminals 27 to
28 for system two (if applicable).
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 shut-
down, 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 suction
pressure falls below the cutout, or for 180 seconds,
whichever comes first
FORM 150.73-NM2
ISSUE DATE: 09/07/2020
JOHNSON CONTROLS
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