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Designating a Standby Chiller
Regardless of whether you’re using Automatic or Fixed sequence ordering, you can designate a chiller as a standby chiller
with the Staging Mode variable. Each standby chiller must have a Sequence Number greater than or equal to all non-
standby chillers for which it is in standby.
If you designate a standby chiller, the CSM does not allow it to operate unless at least one other chiller is off-line. It does
this by checking for an Unavailable Chiller with a Sequence Number lower (or equal) than the Sequence Number of the
standby chiller. Before enabling a standby chiller the CSM also checks to make sure that the Unavailable Chiller was not
recently disabled and unavailable due to internal cycle timers. If the chiller was disabled normally, the CSM will wait for
the cycle timers to clear on the recently disabled chiller and enable it again (standby chiller stays disabled).
CAUTION
An off-line chiller may be operational if it becomes unavailable as a result of (1) losing communications with the
CSM or (2) being locally enabled. In these instances, the standby chiller could start, making possible a situation in
which all chillers are running at the same time. Also, if the off-line chiller that made it possible for the Standby
chiller to be enabled comes back on-line, it will be enabled normally if conditions require further cooling capacity.
The Standby chiller will always be the Next-OFF chiller. This means that at any time after the Standby chiller is enabled,
as soon as conditions exist that will allow a stage-down, the Standby chiller will always be the first to be disabled.
NOTE: It may be necessary that a standby chiller absolutely not run when all other chillers are running due to maximum
flow capacity in the system piping or another reason. For standby centrifugal chillers only, configure the standby
centrifugal chiller to be in row 2 with all other chillers except one. The capacity of row 2 will need to be designated to be
less than the capacity of row 1. Contact the McQuay Controls Support Group at 866-4MCQUAY(866-462-7829) for
comprehensive setup assistance for determining the designated capacity of row 2.
To designate a standby chiller
• Set the Staging Mode of the standby chiller to “Standby”.
• Set the Sequence Number of the Standby chiller greater than or equal to the Sequence Number of all chillers you want
it to be standby to.
Sequencing Chillers with Series-Piped Evaporators
Chiller systems with chillers piped in a Series/Parallel Configuration require slightly different staging logic. There are
situations when both series-piped chillers must be enabled together on a stage-up, and disabled together on a staged-down.
Load balancing will typically be used when the CSM is controlling series-piped centrifugal chillers. Place each pair of
series-piped chillers in a unique Load Balancing Group so that those two chiller loads are balanced.
Applications of series-piped chillers almost always consist of centrifugal chillers and the CSM was designed to control
series-piped centrifugal chillers. If you have a chiller system consisting of series-piped screw or scroll chillers contact the
McQuay Applications Group to see if it is possible to control your application with the CSM.
The operator must inform the CSM that chillers are piped in series for this logic to occur. This is done on the Chiller Setup
screen by entering the chiller number of the other chiller making up the series-piped pair into the “This Chiller Series Piped
With” property of both chillers. Filling in this property also allows the CSM to properly calculate water flow rates when
series-piped chillers are present. These flow rates are used to determine the flow through the decoupler line and the
primary flow that will be lost when the Next Off chiller is disabled.
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