YORK
Start-Up & Operation Guide | 5587001-YSG-A-520
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Operation
• The unit controller monitors the operating
conditions of the variable speed drive com-
pressor. If the variable speed drive compres-
sor speed operates too slow for too long, the
oil return sequence is started.
• If an oil level sensor is installed, the oil return
sequence is started when the oil level drops
below the minimum value.
Speed Limits
The unit controller adjusts the minimum and maxi-
mum speeds of the variable speed drive compres-
sors based the following conditions:
• Oil management
• Overall cooling demand
• Saturated discharge temperature
• Saturated suction temperature
The unit controller continuously monitors the
above conditions/values and provides the appro-
priate speed signal to the variable speed drive
compressor VFD.
Compressor and Refrigeration Circuit Safeties
The rooftop unit has numerous factory installed pres-
sure switches, pressure transducers, and temperature
sensors that protect the compressors and the refrig-
eration circuits.
Compressor Safety Chain and Low Pressure
Cutout Switch
Every refrigeration circuit has a safety chain and a
low pressure cutout (LPCO) switch.
The safety chain consists of a high pressure cut-
out (HPCO) switch and compressor motor protec-
tion module (if applicable).
• The compressor motor protection modules
are installed at the discretion of the compres-
sor manufacturer based on the horsepower
of the compressor. Please refer to
for a list of compressors with motor protec-
tion modules
• Compressors without compressor motor pro-
tection modules have internal overloads in
the motor windings. The internal overloads
are not a part of the safety chain
The LPCO switch is not part of the safety chain. It
is a separate input to the appropriate condenser
board.
The LPCO is ignored for the first 60 seconds of
compressor operation to allow for pressure sta-
bilization
Refer to the following cutout and cut-in pressures:
•
HPCO:
Cutout = 625 psig ± 25
Cut-in = 500 psig ± 25
•
LPCO:
Cutout = 50 psig ± 5
Cut-in = 70 psig ± 5
Compressor Safety Chain and LPCO Safety
Trips and Lockouts
The unit controller continuously monitors the com-
pressor safety chain and LPCO.
When either the safety chain or LPCO open during
operation, the affected refrigeration circuit shuts
down and is logged into memory as Safety Trip 1,
Safety Chain or LPCO. At the same time, the unit
controller starts a 120 minute lockout timer.
When the open safety resets, compressor opera-
tion resumes.
When a second safety chain or LPCO trip oc-
curs, it is logged as Safety Trip 2, Safety Chain
or LPCO.
When the open safety resets a second time, com-
pressor operation resumes
A third safety chain or LPCO trip within the 120
minute timer sequence causes the affected refrig-
eration circuit to enter a safety lockout.
After a safety lockout occurs, it must be manually
reset through the unit controller.
Resetting a Compressor Safety Lockout
A compressor safety lockout must be manually
cleared using the user interface.
The reset function can be found in the
Summary >
Unit > Reset Lockouts
menu.
