Johnson Controls
175
Form 150.72-ICOM6
Issue date: 12/19/2023
8
Unit operating sequence
The operating sequence described below relates to op-
eration on a hot water start after power has been ap-
plied, such as start-up commissioning. When a com-
pressor 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 sched-
uling the chiller OFF, and temperature demand
must be present.
2. When power is applied to the system, the micro-
processor will start a 2 minute timer. This is the
same timer that prevents an instantaneous start af-
ter a power failure.
3. At the end of the 2 minute timer, the micropro-
cessor will check for cooling demand. If all con-
ditions allow for start, a compressor on the lead
system will start and the liquid line solenoid will
open. Coincident with the start, the anti-coinci-
dent timer will be set and begin counting down-
ward 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” sys-
tem. A new lead/lag assignment is made whenever
all systems shut down.
4. Several seconds after the compressor starts, that
systems first condenser fan will be cycled ON
(outdoor air temperature more than 25°F (-4°C)
or discharge pressure). See
condenser fan cycling.
5. After 1 minute of compressor run time, the next
compressor in sequence will start when a sys-
tem 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 five min
-
utes. See the section on Capacity Control for a
detailed explanation of system and compressor
staging.
7. As the load decreases below setpoint, the com-
pressors 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 set-
for a detailed explana-
tion.
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 and the last compressor
will be allowed to run until the suction pressure
falls below the suction pressure cutout or for 180
seconds, whichever comes first.
Capacity control
To initiate the start sequence of the chiller, all run per-
missive inputs must be satisfied (flow/remote start/stop
switch), and no chiller or system faults exist.
The first phase of the start sequence is initiated by the
Daily Schedule Start or any Remote Cycling Device. If
the unit is shut down on the daily schedule, the chilled
water pump contacts (Terminals 23 and 24 of XTBC2)
will close to start the pump when the daily schedule
start time has been reached. Once flow has been es-
tablished and the flow switch closes, capacity control
functions are initiated, if the remote cycling contacts
wired in series with the flow switch are closed.
It should be noted that the chilled water pump contacts
(Terminals 23 and 24 of XTBC2) are not required to
be used to cycle the chilled water pump. However, in
all cases the flow switch must be closed to allow unit
operation.
The control system will evaluate the need for cooling
by comparing the actual leaving or return chilled liquid
temperature to the preferred setpoint, and regulate the
leaving or return chilled liquid temperature to meet that
setpoint.
Section 8: Unit operation
