Unit Operating Sequence - York YLAA0285SE Manual

SECTION 6 – COMMISSIONING
The operating sequence described below relates to
operation 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 one
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 micro-
processor will start a two minute timer. This is the
same timer that prevents an instantaneous start after
a power failure.
3. At the end of the two minute timer, the microproces-
sor 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.
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 shutdown.
136

UNIT OPERATING SEQUENCE

4. Several seconds after the compressor starts, the
systems first condenser fan will be cycled on
(outdoor air temperature greater than 25 °F (-4
°C) or discharge pressure). See the "Operating
Controls" section of this IOM for details concern-
ing condenser fan cycling.
5. After one minute of compressor run time, the
next compressor 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 five min-
utes. Refer to the "Capacity Control" section of
this IOM 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 "Capacity Control" section
of this IOM 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 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.
FORM 150.72-NM3 (1020)
ISSUE DATE 10/05/2020
JOHNSON CONTROLS