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LEAD/LAG OPERATION — The PIC control has the ca-
pability to operate 2 chillers in the lead/lag mode. It also has
the additional capability to start a designated standby
chiller when either the lead or lag chiller is not operating
and capacity requirements are not being met. The lead/lag
option operates in CCN mode only. If any other chiller con-
figured for lead/lag is set to the LOCAL or OFF modes, it
will be unavailable for lead/lag operation.
NOTE: Lead/lag configuration is viewed and edited on the
LEAD/LAG screen, accessed from the EQUIPMENT CON-
FIGURATION table of the SERVICE menu. See Table 2,
Example 7. Lead/lag status during chiller operation is viewed
on the MAINT04 screen, accessed from the CONTROL AL-
GORITHM STATUS table. See Table 2, Example 14.
Lead/Lag Chiller Configuration and Operation — A chiller
is designated the lead chiller when the LEAD/LAG
SELECT parameter for that chiller is set to 1 on the LEAD/
LAG CONFIGURATION screen. A chiller is designated the
lag chiller when the LEAD/LAD SELECT parameter for that
chiller is set to 2. A chiller is designated the standby chiller
when the LEAD/LAG SELECT parameter for that chiller is
set to 3. Setting the LEAD/LAG SELECT parameter to 0 dis-
ables the lead/lag function in that chiller.
To configure the LAG ADDRESS parameter on the LEAD/
LAG CONFIGURATION screen, always use the address of
the other chiller on the system. Using this address makes it
easier to rotate the lead and lag chillers.
If improper address assignments are entered for the LAG
ADDRESS and STANDBY ADDRESS parameters, the
lead/lag is disabled and an alert (!) message displays on the
LID. For example, if the lag chiller’s address matches the
lead chiller’s address, the lead/lag function is disabled and
an alert (!) message displays. The lead/lag maintenance screen
(MAINT04) displays the message INVALID CONFIG in the
LEAD/LAG CONFIGURATION and CURRENT MODE fields.
The lead chiller responds to normal start/stop controls such
as occupancy schedule, forced start/stop, and remote start
contact inputs. After completing start-up and ramp loading,
the PIC evaluates the need for additional capacity. If addi-
tional capacity is needed, the PIC initiates the start-up of the
chiller configured at the lag address. If the lag chiller is faulted
(in alarm) or is in the OFF or LOCAL modes, then the chiller
at the standby address (if configured) is requested to start.
After the second chiller is started and is running, the lead
chiller monitors conditions and evaluates whether the ca-
pacity has been reduced enough for the lead chiller to sus-
tain the system alone. If the capacity is reduced enough for
the lead chiller to sustain the control point temperatures alone,
then the operating lag chiller is stopped.
If the lead chiller is stopped in CCN mode for any reason
other than an alarm (*) condition, then the lag and standby
chillers are stopped. If the configured lead chiller stops for
an alarm condition, then the configured lag chiller takes the
lead chiller’s place as the lead chiller and the standby chiller
serves as the lag chiller.
If the configured lead chiller does not complete the start-up
before the PRESTART FAULT TIMER (a user configured pa-
rameter on the LEAD/LAG screen) elapses, then the lag chiller
is started and the lead chiller shuts down. The lead chiller
then monitors the request to start from the acting lead chiller.
The pre-start fault timer is initiated at the time of a start re-
quest. This timer’s function is to provide a time-out if there
is a pre-start alert condition that prevents the chiller from
starting in a timely manner.
If the lag chiller does not achieve start-up before the
pre-start fault time elapses, then the lag chiller is stopped
and the standby chiller is requested to start, if configured
and ready.
Standby Chiller Configuration and Operation — The con-
figured standby chiller is identified as such by having its LEAD/
LAG SELECT parameter assigned a value of 3. The standby
chiller can only operate as a replacement for the lag chiller
if one of the other two chillers is in an alarm (*) condition
(as indicated on the LID panel). If both lead and lag chillers
are in an alarm (*) condition, the standby chiller defaults to
operate in CCN mode based on its configured occupancy sched-
ule and remote contacts input.
Lag Chiller Start-Up Requirements — Before the lag chiller
can be started, the following conditions must be met.
1. Lead chiller ramp loading must be completed.
2. Lead chiller’s chilled water temperature must be
greater than the WATER/BRINE CONTROL POINT (STA-
TUS01 screen) plus half the WATER/BRINE DEAD-
BAND (SERVICE1 screen).
NOTE: The chilled water temperature sensor may be the
leaving chilled water sensor, the return water sensor, the
common supply water sensor, or the common return wa-
ter sensor, depending on which options are configured and
enabled.
3. Lead chiller ACTIVE DEMAND LIMIT (STATUS01
screen) value must be greater than 95% of full load amps.
4. Lead chiller temperature pulldown rate (TEMP PULL-
DOWN DEG/MIN on the CONFIG screen) of the chilled
water temperature is less than 0.5° F (0.27° C) per minute.
5. The lag chiller status indicates it is in CCN mode and is
not faulted. If the current lag chiller is in an alarm con-
dition, then the standby chiller becomes the active lag chiller,
if it is configured and available.
6. The configured time for the LAG START TIMER param-
eter has elapsed. The lag start timer starts when the lead
chiller ramp loading is completed. The LAG START TIMER
parameter is on the LEAD/LAG screen, which is ac-
cessed from the EQUIPMENT CONFIGURATION table.
See Table 2, Example 7.
When all the above requirements have been met, the lag
chiller is forced to a STARTUP mode. The PIC control then
monitors the lag chiller for a successful start. If the lag chiller
fails to start, the standby chiller, if configured, is started.
Lag Chiller Shutdown Requirements — The following con-
ditions must be met in order for the lag chiller to be stopped.
1. Lead chiller COMPRESSOR MOTOR LOAD (STA-
TUS01 screen) value is less than the lead chiller percent
capacity plus 15%. See STATUS01 screen or Table 2,
Example 1.
NOTE: Lead chiller percent capacity = 100 – LAG PER-
CENT CAPACITY.
The LAG PERCENT CAPACITY value is configured on
the LEAD/LAG CONFIGURATION screen.
2. The lead chiller chilled water temperature is less than
the WATER/BRINE CONTROL POINT plus 1/2 of the
WATER/BRINE DEADBAND. The WATER/BRINE DEAD-
BAND parameter is on the SERVICE1 screen. See
Table 2, Example 8.
3. The configured lag stop time (LAG STOP TIMER param-
eter on the LEAD/LAG CONFIGURATION screen) has
elapsed. The lag start time starts when the LEAVING
CHILLED WATER temperature is less than the WATER/
BRINE CONTROL POINT plus 1/2 of the WATER/
BRINE DEADBAND, and the lead chiller COMPRESSOR
MOTOR LOAD is less than the lead chiller percent ca-
pacity plus 15%. The lag stop timer is ignored if the chilled
water temperature reaches 3° F (1.67° C) below the WATER/
BRINE CONTROL POINT and the lead chiller COM-
PRESSOR MOTOR LOAD value is less than the lead chiller
percent capacity plus 15%.
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