56
Lead/Lag Control — The lead/lag control system auto-
matically starts and stops a lag or second chiller in a 2-chiller
liquid system. A third chiller can be added to the lead/lag
system as a standby chiller to start up in case the lead or lag
chiller in the system has shut down during an alarm condition
and additional cooling is required. Refer to Fig. 27 and 28 for
menu, table, and screen selection information. The output is
2 mA when the chiller is not running.
NOTE: The lead/lag function can be configured on the
LEADLAG screen, which is accessed from the SERVICE
menu and EQUIPMENT SERVICE table. See Table 3,
Example 27. Lead/lag status during chiller operation can be
viewed on the LL_MAINT display screen, which is accessed
from the SERVICE menu and CONTROL ALGORITHM
STATUS table. See Table 3, Example 14.
Lead/Lag System Requirements:
• All chillers in the system must have software capable of
performing the lead/lag function.
• Liquid pumps MUST be energized from the PIC III
controls.
• Liquid flows should be constant.
• The CCN time schedules for all chillers must be identi-
cal. The lag chiller set point and demand limit are con-
trolled by lead chiller only and cannot be written to by
an external automation system.
Operation Features:
• 2-chiller lead/lag
• addition of a third chiller for backup
• manual rotation of lead chiller
• load balancing if configured
• staggered restart of the chillers after a power failure
• chillers may be piped in parallel or in series chilled
liquid flow
COMMON POINT SENSOR INSTALLATION — In
all cases lead/lag operation does not require a common point
chilled liquid sensor. Common point sensors (Spare Temp #1
and #2) can be added to the CCM module, if desired. Spare
Temp #1 and #2 are wired to plug J4 terminals 25-26 and 27-28
(J4 lower, respectively). See the Lead/Lag Control Wiring fig-
ures on pages 143 and 144.
NOTE: If the common point sensor option is chosen on a
chilled liquid system, each chiller should have its own com-
mon point sensor installed. Each chiller uses its own common
point sensor for control when that chiller is designated as the
lead chiller. The PIC III controls cannot read the value of com-
mon point sensors installed on the other chillers in the chilled
liquid system.
If leaving chilled liquid control (ECL CONTROL OPTION
is set to 0 [DSABLE], TEMP_CTL screen) and a common
point sensor is desired (COMMON SENSOR OPTION in
LEADLAG screen selected as 1) then the common point
temperature sensor is wired in Spare Temp #1 position on the
CCM.
If the entering chilled liquid control option (ECL CON-
TROL OPTION) is enabled (configured in TEMP_CTL
screen) and a common point sensor is desired (COMMON
SENSOR OPTION in LEADLAG screen selected as 1) then
the sensor is wired in Spare Temp #2 position on the CCM.
When installing chillers in series, a common point sensor
should be used. If a common point sensor is not used, the
leaving chilled liquid sensor of the upstream chiller must be
moved into the leaving chilled liquid pipe of the downstream
chiller.
If return chilled liquid control is required on chillers piped
in series, the common point return chilled liquid sensor should
be installed. If this sensor is not installed, the return chilled
liquid sensor of the downstream chiller must be relocated to the
return chilled liquid pipe of the upstream chiller.
To properly control the common supply point temperature
sensor when chillers are piped in parallel, the liquid flow path
through the shutdown chillers must be isolated so no liquid
bypass around the operating chiller occurs. The common point
sensor option must not be used if liquid bypass around the
operating chiller is occurring.
CHILLER COMMUNICATION WIRING — Refer to the
chiller’s Installation Instructions, Carrier Comfort Network
®
Interface section for information on chiller communication
wiring.
LEAD/LAG OPERATION — The PIC III control system not
only has the ability to operate 2 chillers in lead/lag, but it can
also start a designated standby chiller when either the lead or
lag chiller is faulted and capacity requirements are not met. The
lead/lag option only operates when the chillers are in CCN
mode. If any other chiller configured for lead/lag is set to the
LOCAL or OFF modes, it will be unavailable for lead/lag
operation.
Lead/Lag Chiller Configuration and Operation
• A chiller is designated the lead chiller when its LEAD/
LAG CONFIGURATION value on the LEADLAG
screen is set to “1.”
• A chiller is designated the lag chiller when its LEAD/
LAG CONFIGURATION value is set to “2.”
• A chiller is designated as a standby chiller when its
LEAD/LAG CONFIGURATION value is set to “3.”
• A value of “0” disables the lead/lag designation of a
chiller.
To configure the LAG ADDRESS value on the LEADLAG
screen, always enter the address of the other chiller on the
system. For example, to configure chiller A, enter the address
for chiller B as the lag address. To configure chiller B, enter the
address for chiller A as the lag address. This makes it easier to
rotate the lead and lag chillers.
If the address assignments in the LAG ADDRESS and
STANDBY ADDRESS parameters conflict, the lead/lag func-
tion is disabled and an alert (!) message displays. For example,
if the LAG ADDRESS matches the lead chiller’s address, the
lead/lag will be disabled and an alert (!) message displayed.
The lead/lag maintenance screen (LL_MAINT) displays the
message INVALID CONFIG in the LEAD/LAG: CONFIGU-
RATION and CURRENT MODE fields. Refer to Table 7.
Table 7 — Invalid Lead/Lag Addresses
The lead chiller responds to normal start/stop controls such
as the occupancy schedule, a forced start or stop, and remote
start contact inputs. After completing start-up and ramp load-
ing, the PIC III module evaluates the need for additional capac-
ity. If additional capacity is needed, the PIC III module 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, 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
LEAD/LAG
CONFIGURATION
(IN LEAD/LAG
SCREEN)
INVALID CONDITIONS
1 (Lead)
Local Address (Lead) =
Lag Address
Standby Chiller Option = Enable and
Local Address (Lead) = Standby Address
Standby Chiller Option = Enable and
Lag Address = Standby Address
Local Address (Lead) =
Lag Address
2 (Lag)
Standby Chiller Option = Enable and
Local Address (Lag) = Standby Address
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