YORK INTERNATIONAL
FORM 150.63-NM1
95
The operating sequence described below relates to op-
eration after power has been applied, such as start-up
commissioning. When a compressor starts, internal tim-
ers limit the minimum time before another compressor
can start to 1 minute.
1. For the condensing unit to run, any remote cycling
contacts must be closed, the Daily Schedule must
not be scheduling the condensing unit off, the air
proving switch(es) must be closed, and a load 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 microproces-
sor will check for cooling demand. If all conditions
allow for start, the first compressor on the lead sys-
tem will start and the liquid line solenoid will open.
The compressor with the least run time in that sys-
tem will be the first to start. Coincident with the
start, the anti-coincident timer will be set and be-
gin 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 sys-
tems first condenser fan will be cycled on (outdoor
air temperature > 25°F (-3.89°C)). See the section
on Operating Controls for details concerning con-
denser fan cycling.
5. After 3 minutes of compressor run time, the next
compressor in sequence will start when a system
has to load. This compressor will be the one with
the least run time that is currently not running in
that system. Additional compressors will be started
at 150 to 180 second intervals as needed to satisfy
(discharge temperature or suction pressure)
setpoint.
6. If demand requires, the lag system will cycle on
with the same timing sequences as the lead sys-
tem. Refer to the section on Capacity Control for a
detailed explanation of system and compressor stag-
ing.
7. As the load decreases below setpoint, the compres-
sors will be shut down in sequence. This will oc-
cur at intervals of either 60 or 30 seconds depend-
ing on the mode of control as compared to setpoint.
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. On a non-
safety, non-unit switch shutdown, the LLSV will
be turned off, and the last compressor will be al-
lowed to run until the suction pressure falls below
the suction pressure cutout or for 180 seconds,
which ever comes first.
UNIT OPERATING SEQUENCE
2
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