McQuay AGS 225D, Operating Manual

Page: 11 / 82

OM AGSD-1 11
Sequence of Operation
Figure 4, Unit Sequence of Operation (see Figure 4 for circuit sequence of operation)
Is unit enabled?
Is flow present?
Evaporator pump output on
Wait for chilled water loop to
recirculate.
Is there enough load to
start chiller?
Start first circuit.
Yes
Yes
Yes
No
No
Unit power up
Unit in Off state
No
The chiller may be disabled via the unit switch, the remote switch, the keypad
enable setting, or the BAS network. In addition, the chiller will be disabled if both
circuits are disabled, either because of an alarm or the circuit pumpdown switch on
each circuit, or if there is a unit alarm. If the chiller is disabled, the unit status
display will reflect this and also show why it is disabled.
If the unit switch is off, the unit status will be Off:Unit Switch . If the chiller is
disabled due to network command, the unit status will be Off:BAS Disable . When
the remote switch is open, the unit status will be Off:Remote Switch . When a unit
alarm is active, the unit status will be Off:Unit Alarm . In cases where no circuits
are enabled, the unit status will be Off:All Cir Disabled .
If the chiller is enabled, then the unit will be in the Auto state and the evaporator
water pump output will be activated.
After establishing flow, the chiller will wait some time to allow the chilled water loop
to recirculate for an accurate reading of the leaving water temperature. The unit
status during this time is
Auto:Evap Recirc .
The chiller will then wait for the flow switch to close, during which time the unit
status will be
Auto:Wait for flow .
Keep pump output on while
unit is enabled.
The chiller is now ready to start if enough load is present. If the LWT is not high
enough to start, the unit status will be
Auto:Wait for load .
If the LWT is high enough to start, the unit status will be Auto . A compressor can
start at this time.
The first circuit to start is generally the available circuit with the least number of
starts, or the lowest numbered circuit if there is a tie. This circuit will go through its
start sequence at this point.