Unit installation

54 IOMM ACZ/AGZ
Unoccupied Mode
When the remote start / stop input is opened, the unoccupied mode is initiated. The chiller will
maintain a higher leaving water temperature setpoint (default 70°F) if the chilled water circulation
is maintained. The unoccupied leaving water setpoint can be changed from the Zone Terminal.
Soft Start
Soft loading limits the number of available stages when the unit is started to prevent excessive
power consumption and possible overshoot of the leaving water temperature set point. Soft loading
is in effect whenever the unit is started from an “off” cycle. When stage up starts, the controller
starts a countdown timer to indicate how long the unit has been in the cool stage mode. The number
of stages allowed during soft loading is determined by the Soft Start Capacity. The duration of the
soft load sequence is determined by the Soft Start Timer. If the Soft Start Timer is set to zero, no
soft loading will take place. The Zone Terminal is required to change the Soft Start values.
Compressor Staging
The AGZ Scroll chiller has two refrigerant circuits and each circuit has two scroll compressors.
When the first stage of cooling is required, the #1 refrigerant circuit will start. This circuit will be
the first to start for the next ten Starts. The Global UNT controller will then select the #2
refrigerant circuit to start first for the next ten starts. This cycle will continue and balance the
number of starts between the refrigerant circuits. This is an automatic lead / lag control.
The compressors are always started in a fixed order. Refrigerant circuit #1 has compressor #1 and
compressor #3. Compressor #1 will always start first on refrigerant circuit #1. Refrigerant circuit
#2 has compressor #2 and compressor #4. Compressor #2 will always start first on refrigerant
circuit #2.
There are four stages of cooling capacity on the Global Scroll chiller unit. The first stage of cooling
will be the first compressor of the lead refrigerant circuit. The second stage of cooling will be the
first compressor of the lag refrigerant circuit. Cooling stage three will always be compressor #3 and
cooling stage four will always be compressor #4.
Stage down is the reverse of the above paragraph.
Pumpdown Control
The AGZ units have a one time pump down. The pump down is activated when a circuit’s liquid
line solenoid valve is de-energized. After the circuit’s liquid line solenoid valve is de-energized, the
operating compressor(s) on that refrigerant circuit will operate until the low pressure switch opens
and stops that compressor(s). The compressor will not cycle on if the evaporator pressure exceeds
the low pressure cut-in value. The compressor will start if the Global UNT controller calls for
cooling.
The Pumpdown and Stop switch will open its respective circuit’s digital input to the Global UNT
controller. This will open the associate circuit’s output and open the circuit’s solenoid valve. If a
compressor is operating on that circuit, the compressor will operate until the low pressure switch
stops the compressor. If a compressor is not running, the switch does not activate a pumpdown for
that circuit but will not allow the circuit to stage on and operate.
Certain alarm conditions will not permit chiller pumpdown. These alarm conditions are:
• High Condenser Pressure
• Bad Evap Pressure Sensor
• Compressor Motor Protect
• Freezestat Alarm