Specifications

57
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. 18 and 19 for
menu, table, and screen selection information.
NOTE: The lead/lag function can be configured on the LEAD-
LAG screen, which is accessed from the SERVICE menu and
EQUIPMENT SERVICE table. See Table 3, Example 23.
Lead/lag status during chiller operation can be viewed on the
LL_MAINT display screen, which is accessed from the SER-
VICE menu and CONTROL ALGORITHM STATUS table.
See Table 3, Example 13.
Lead/Lag System Requirements:
• all chillers in the system must have software capable of
performing the lead/lag function
• water pumps MUST be energized from the PIC II
controls
• water flows should be constant
• the CCN time schedules for all chillers must be identical
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 water
flow
COMMON POINT SENSOR USAGE AND INSTALLA-
TION — Refer to 19XR,XRV Lead Lag Schematics, Appen-
dix B. Lead/lag operation does not require a common point
chilled water sensor. Common point sensors (Spare Temp#1
and #2) may be added as described below, if desired.
If using leaving chilled water control (ECW CONTROL
OPTION is set to 0 [DSABLE] on the TEMP_CTL screen) and
a common point sensor is desired (COMMON SENSOR OP-
TION in LEADLAG screen selected as 1) then the sensor is
wired into the Spare Temp #1 position on the CCM (terminals
J4-25 and J4-26).
If the entering chilled water control option is enabled (ECW
CONTROL OPTION is selected to 1 [configured in the
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 (terminals J4-27 and J4-28).
NOTE: If the common point sensor option is chosen on a
chilled water system, each chiller should have its own common
point sensor installed. Each chiller uses its own common point
sensor for control when that chiller is designated as the lead
chiller. The PIC II cannot read the value of common point sen-
sors installed on the other chillers in the chilled water system.
When installing chillers in series, either a common point
sensor should be used (preferred), or the LEAVING CHILLED
WATER sensor of the upstream chiller must be moved into the
leaving chilled water pipe of the downstream chiller. In this ap-
plication the COMMON SENSOR OPTION should only be en-
abled for the upstream chiller if that chiller is configured as the
Lead.
If ENTERING CHILLED WATER control is required on
chillers piped in series, either a common point sensor should be
used (preferred), or the ENTERING CHILLED WATER sensor
of the downstream chiller must be relocated to the ENTERING
CHILLED WATER pipe of the upstream chiller. In this applica-
tion the COMMON SENSOR OPTION should only be enabled
for the downstream chiller if that chiller is configured as the
Lead. Note that ENTERING CHILLED WATER control is not
recommended for chillers installed in series due to potential
control stability problems.
To properly control the LEAVING CHILLED WATER TEM-
PERATURE when chillers are piped in parallel, the water flow
through the shutdown chiller(s) should be isolated so that no
water bypass around the operating chiller occurs. However, if
water bypass around the operating chiller is unavoidable, a
common point sensor in the mixed LEAVING CHILLED WA-
TER piping should be provided and enabled for the Lead
chiller.
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 II 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 op-
tion only operates when the chillers are in CCN mode. If any
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
LEADLAG: CONFIGURATION value on the LEAD-
LAG screen is set to “1.”
• A chiller is designated the lag chiller when its
LEADLAG: CONFIGURATION value is set to “2.”
• A chiller is designated as a standby chiller when its
LEADLAG: CONFIGURATION value is set to “3.”
• A value of “0” disables the lead/lag designation of a
chiller.This setting should also be used when “normal”
operation without regard to lead/lag rules is desired (in
LOCAL or CCN mode).
When configuring the LAG ADDRESS value on the
LEADLAG screen of chiller “A” enter the address of the chill-
er on the system which will serve as lag when/if chiller “A” is
configured as lead. For example, if the user is configuring chill-
er A, enter the address for chiller B as the lag address. If you
are configuring chiller B, enter the address for chiller A as the
lag address. This makes it easier to rotate the lead and lag chill-
ers. Note that only the lag and standby chiller addresses speci-
fied in the configured lead chiller's table are relevant at a given
time.
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 LEADLAG: CONFIGU-
RATION and CURRENT MODE fields. See Table below.
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 II evaluates the need for additional capacity. If ad-
ditional capacity is needed, the PIC II 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 whether the ca-
pacity has been reduced enough for the lead chiller to sustain
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.
LEAD/LAG
CONFIGURATION
INVALID
CONDITIONS
1 (Lead)
Local Addres (Lead) = Lag Address
Standby Chiller Option = Enable AND Local
Address (Lead) = Standby Address
Standby Chiller Option = Enable AND Local
Address (Lag) = Standby Address
2 (Lag)
Local Addres (Lead) = Lag Address
Standby Chiller Option = Enable AND Local
Address (Lag) = Standby Address