JOHNSON CONTROLS
69
FORM 150.62-NM6 (103)
The operating sequence described below relates to op-
eration on a hot water start after power has been applied,
such as start-up commissioning. When a compressor
starts, internal timers limit the minimum time before
another compressor can start to 1 minute.
1. For the chiller system to run, the Flow Switch must
be closed, any remote cycling contacts must be
closed, the Daily Schedule must not be scheduling
the chiller off, and temperature demand must be
present.
2. When power is applied to the system, the micropro-
cessor will start a 2 minute timer. This is the same
timer that prevents an instantaneous start after a
power failure.
3. At the end of the 2 minute timer, the microprocessor
will check for cooling demand. If all conditions allow
for start, a compressor on the lead system will start
and the liquid line solenoid will open or the EEV
will begin to operate (EEV equipped chillers only).
Coincident with the start, the anti-coincident timer
will be set and begin counting downward from “60”
seconds to “0” seconds.
If the unit is programmed for Auto Lead/Lag, the
system with the shortest average run-time of the
compressors will be assigned as the “lead” system.
A new lead/lag assignment is made whenever all
systems shut down.
4. Several seconds after the compressor starts, that
systems rst condenser fan will be cycled on (out-
door air temperature > 25°F (-4°C) or discharge
pressure). See the section on Operating Controls
for details concerning condenser fan cycling.
YCAL0090 – YCAL0124 cycle fans on discharge
pressure only.
5. After 1 minute of compressor run time, the next com-
pressor in sequence will start when a system has to
load. Additional compressors will be started at 60
second intervals as needed to satisfy temperature
setpoint.
6. If demand requires, the lag system will cycle
on with the same timing sequences as the lead
system after the lead system has run for ve min-
utes. Refer to the section on Capacity Control for
a detailed explanation of system and compressor
staging.
7. As the load decreases below setpoint, the compres-
sors will be shut down in sequence. This will occur
at intervals of either 60, 30, or 20 seconds based
on water temperature as compared to setpoint, and
control mode. See the section on Capacity Control
for a detailed explanation.
8. When the last compressor in a “system” (two or
three compressors per system), is to be cycled off,
the system will initiate a pump-down. Each “system”
has a pump-down feature upon shut-off. On a non-
safety, non-unit switch shutdown, the LLSV will be
turned off or the EEV will close (EEV equipped chill-
ers only), and the last compressor will be allowed
to run until the suction pressure falls below the suc-
tion pressure cutout or for 180 seconds, whichever
comes rst.
UNIT OPERATING SEQUENCE
1
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