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Cheatography

The Art & Science of Point Selection Cheat Sheet (DRAFT) by [deleted]

Point Selection

This is a draft cheat sheet. It is a work in progress and is not finished yet.

Introd­uction

BAS 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)?

Point selection “science”:

1. 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.
2. 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 below).
3. From the diagram identify each of the system’s components to be controlled (e.g., “Supply Fan,” “Cooling Coil,” “Mixed Air Dampers,” etc.).
4. 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.
5. 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”:

1. 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 experi­ence.
2. 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.
3. 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.
4. 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.