McQuay ACZ 010A, Installation, Operation And Maintenance Manual, Page: 29 / 48

IOMM ACZ1 ACZ 010A through 039A 29
Unit Mode Selection
The operating mode of the chiller is set by the Unit Mode Setpoint with options of COOL and TEST.
COOL is the default and is the only standard operating mode. TEST is only available for service
personnel. This mode setting can be altered by the keypad, BAS, and Mode input. Changes to the
Unit Mode Set Point are controlled by two additional setpoints.
•
Available Modes Setpoint: Determines the operational modes available at any time with options
of COOL and TEST.
•
Control Source Setpoint: Determines the source that can change the Unit Mode Set Point with
options of KEYPAD, NETWORK, or SWITCHES.
Low Ambient Start
If SpeedTrol = N, the unit will start in the normal operation. If the SpeedTrol = Y then starting will
follow table below. This step will bypass the “Low Evaporator Pressure” alarm until Low Ambient
Start is completed.
Low Ambient Start Method
Table 14, Low Ambient Start Sequence
Description Occurs When: Action Taken
Check #1
After 15 Seconds after starting Lead Compressor, If the
Evap Press is < (0.48 times the Low Evap Press SP) take
Action or else continue Rapid Stop – See Low Ambient
Re-Start below
Check #2
After 15 Seconds after Check #1, If the Evap Press is <
(0.66 times the Low Evap Press SP) take Action or else
continue Rapid Stop – See Low Ambient
Re-Start below
Check #3
After 15 Seconds after Check #2, If the Evap Press is <
(0.83 times the Low Evap Press SP) take Action or else
continue Rapid Stop – See Low Ambient
Re-Start below
Check #4
After 15 Seconds after Check #3, If the Evap Press is <
Low Evap Pressure SP take Action or else continue in
normal operation Rapid Stop – See Low Ambient
Re-Start below
Low Ambient Restart
If the Evap Pressure fails during the low-ambient start, the controller will wait until the anti-cycle
timers expire then try to restart. It will allow unit to attempt to start 3 times. If the unit starts
successfully, the counter is reset. If it fails to start on the third attempt, the Low Ambient Restart
Alarm Fault (Manual Reset) will be activated.
Automatic Adjusted Limits
The following setpoint ranges will be adjusted automatically based on selected options or mode of
operation.
Low Ambient Lockout Temperature
SpeedTrol Range
SpeedTrol = N
35– 60
°
F
SpeedTrol = Y
-2 – 60
°
F