Unit installation

98 ACZ / AGZ-BM IOMM ACZ/AGZ-3
Compressor Start/Stop Timing
This section defines when a compressor is to start, or stop, and the scenario for doing so.
Required Parameters
1. Number of Compressors/Circuit setpoint
2. Number of compressors running
3. Interstage timer status
4. Stage request input status
Stage Up
IF [Stage Request > Number of Compressors Running
AND Stage Up Timer Expired]
THEN Stage_Up_Now = YES
Stage Down
IF [Stage Request < Number of Compressors Running
AND Stage Down Timer Expires]
THEN Stage_Down_Now = YES
Circuit Capacity Overrides – Limits of Operation
The following conditions shall override the automatic capacity control when the unit is in the COOL
mode. These overrides keep a circuit from entering a condition in which it is not designed to run.
Low Evaporator Pressure
If a circuit is running and the evaporator pressure drops below the Low Evaporator Pressure-Hold
setpoint, no more compressors will be allowed to start on that circuit. This limit shall be active until
the evaporator pressure reaches the hold setpoint plus 8 psi.
If a circuit is running with two or three compressors on and the evaporator pressure drops below the
Low Evaporator Pressure-Unload setpoint, the circuit will begin reducing capacity. If two
compressors are running, one of the running compressors will be stopped. If three compressors are
running, then one compressor will be stopped initially. Ten seconds later, if the pressure has not
risen above the unload setpoint, another compressor will be stopped. The last compressor on a
circuit will not stop due to the unload condition.
High Condenser Pressure
If the discharge pressure rises above the High Condenser Pressure Unload setpoint and more than
one compressor on the circuit is running, the circuit will stage down. One compressor will
shutdown as soon as the pressure rises above the unload setpoint, and if two remain running, then
one more will shut down 10 seconds later if the pressure is still above the unload setpoint. No stage
up will be allowed on the circuit until the condenser pressure drops to the unload setpoint less 100
psi, and the outdoor ambient temperature drops 5 degrees F.
Low Ambient Lockout
If the OAT drops below the low ambient lockout setpoint, then all running circuits 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 degrees F.