IOMM AGZB1 AGZ 010B through 034B 45
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.
Maximum LWT Rate
The maximum rate at which the leaving water temperature can drop is limited by the
Maximum Pulldown Rate setpoint when the unit mode is Cool. If the rate exceeds this
setpoint, no more compressors can be started until the Pulldown Rate is less than the
setpoint. Running compressors will not be stopped as a result of exceeding the maximum
pulldown rate.
Low Ambient Lockout
If the OAT drops below the low ambient lockout setpoint, then the unit will do a normal
stop. Once the lockout has been triggered, no compressors will start until the OAT rises to
the lockout setpoint plus 5°F.
Compressor Control
Compressor Available
A compressor is available to start when the following are true:
• Unit state = auto
• Evap state = run
• Low OAT lockout is not active
• Power up start delay is expired
• No limit events active
• No cycle timers active for the compressor
• Compressor enable setpoint = On
Compressor Start/Stop Timing
This section determines when to start or stop a compressor. There are two separate
functions used, one for staging up and one for staging down.
Stage Up Now
The Stage Up Now flag is set based on the following tests:
If Unit mode = Cool, AND
no compressors are running, AND
LWT error > Start delta + Upper Control Band, AND
Motor Protect Timer is expired, AND
Stage up timer is expired, THEN
Stage Up Now = True
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