Unit installation

34 ACZ 010A through 039A IOMM ACZ1-2
Fan 3 will stop when the condenser pressure drops to the Stage #3 Off setpoint, and fan 2
will stop when the condenser pressure drops to the Stage #2 Off setpoint. Fan 1 will stop
when the pressure drops down to the Stage #1 Off setpoint.
Low OAT Start
In order to avoid low-pressure alarms at startup, low OAT start logic allows for running at
low pressures for a longer time than normal as well as multiple start attempts.
A low OAT start is initiated if the condenser saturated temperature is less than 85°F when
the compressor starts. Once this happens, the circuit is in this low OAT start state for a
time equal to the low OAT start timer setpoint. During this time, the freezestat logic and
the low pressure events are disabled. The absolute limit of 5 psi is still enforced.
At the end of the low OAT start, the evaporator pressure is checked. If the pressure is
greater than or equal to the low evaporator pressure unload setpoint, the start is considered
successful. If the pressure is less than the unload setpoint, the start is not successful and
the compressor will stop. Three start attempts are allowed before tripping on the restart
alarm; so if on the third attempt the start is not successful the restart alarm is triggered.
The restart counter will be reset when either a start is successful or the circuit is off on an
alarm.
Capacity Overrides
The following conditions override the automatic capacity control when the chiller is in
Cool mode only. These overrides keep the unit from entering a condition in which it is not
designed to run.
Low Evaporator Pressure
If the evaporator pressure drops below the Low Evaporator Pressure Hold setpoint, the Low
Evaporator Pressure Inhibit event is triggered. This can occur with either one or two
compressors running. When triggered, the second compressor will not be allowed to start if
only one is currently running. If both compressors are already running, no action is taken.
If the evaporator pressure drops below the Low Evaporator Pressure Unload setpoint, the
Low Evaporator Pressure Unload event is triggered. This can only occur when both
compressors are running. When triggered, one compressor is shut off.
These events are logged to an event log when they occur. Both remain active until the
evaporator pressure rises 5 psi above the hold setpoint or both compressors are off.
High Condenser Pressure
If the discharge pressure rises above the High Condenser Pressure Unload setpoint and both
compressors are running, the High Condenser Pressure Unload event is triggered. One
compressor will be shut off when this occurs.
This event will also be logged to an event log when it occurs. It will remain active until the
condenser pressure drops 100 psi below the unload setpoint. While active, the second
compressor cannot turn back on.