point selection is one of the most important aspects of BAS design (the
other being the sequence of operation). Yet if two designers
developed a point list for the same project there would be many
differences. Will both lists work well (that depends on the rigor
of the “science” that was applied to the point list selection) and, if
so, will the difference be reflected in the quality of the BAS’s
control (that depends on the “art” behind the point list selection)?
The Art and Science of Point Selection
Point selection “science”:
- Start with a schematic diagram of the system to be
controlled. It could be a sketch or a “one-line” diagram
developed for the mechanical design, or even just a mental image.
- Determine what safeties and/or factory controls come with the
equipment. Are these safeties/controls required in lieu of using
the BAS? Boiler or chiller factory safeties must be used, some
factory-provided controls are fine if left in place (e.g., cooling
tower sump level controls) while others can make the BAS’s execution of
the control sequence more difficult (e.g., an OA flow sensor with
integral OA damper control). The point list should only include
factory-provided safeties if they to be monitored by the BAS or if the
factory controls will be integrated to the BAS (via “objects” discussed
- From the diagram identify each of the system’s components to be
controlled (e.g., “Supply Fan,” “Cooling Coil,” “Mixed Air Dampers,”
- Be mindful of how you want/need to control the system. It
may be helpful to develop an outline of the sequence of operation if
you can’t do this in your head.
- Start the point list by listing out each component. Look at
each component on the schematic and ask yourself what inputs &
outputs are needed to control it. All modulating outputs (e.g.,
AHU coil valve) requires an associated input, motor start/stop outputs
should have associated status inputs, etc.
Point selection “art”:
- Point list
development is iterative with that of the sequence (i.e., neither can
be considered “complete” until the other is complete and vice
versa). When to end the iterations involves judgment based on
- Decide what functions should be controlled by points vs.
“objects” via data communications. The point list needs to
clearly differentiate between points (i.e., each connected by dedicated
analog-signaling wiring) vs. via digitally communicated objects (i.e.,
via BACnet). VFD’s are a perfect example since they typically
come standard with both point connections along with BACnet
communications. I believe functions critical to the BAS sequence
should be connected via points (e.g., start/stop, speed control, and
motor status via a current switch at the motor not via a VFD
connection). Other objects are generally not critical to the
sequence and can be communicated digitally. Which approach to
take with connections for HOA and bypass switches’ positions (if
needed) depends on how critical these are to the sequence.
- It is important that safeties (e.g., freezestats) be directly
hard-wired to turn off the associated equipment (e.g., a motor
starter). Wiring a safety via the BAS provides too much
opportunity to override its function. However, you can choose to
also monitor safeties from the BAS, though knowing which safety has
tripped a motor off may not be critical to the BAS sequence.
- Many other inputs (or objects) can be connected (or communicated)
to a BAS for monitoring and/or troubleshooting purposes. For
example, I would consider an AHU’s MAT to be a useful
monitoring/troubleshooting point even if not critical to the sequence
(while RAT is usually less useful). The monitoring of equipment
energy consumption can be important depending on whether the building
owner’s needs and goals. Owner input on these “optional” points
should be taken into account here.
Point selection has become more
challenging over the years due to more-complicated systems (with more
control & safeties required), more factory-provided controls, and
the availability of BACnet connections to many system components.
But does the industry still have enough time, patience and money for
the “art” needed to make a point list work as best as is possible?
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