CAD Design: What to Include, What to Exclude
BAS design drawings: A CAD-intensive design is expensive
This column ends my tenure as the regular author of this column (thanks for reading!). It also completes my “Back to BASics” series that has focused on the fundamentals of BAS functionality and design. Previously, I stated that the two most important elements of a design are the sequence of operation and point list, but that a specification is also necessary.
The final key element to a BAS design is the drawings. BAS information is typically shown on separate, dedicated drawings that are included as part of the design’s mechanical drawings; however, BAS information can also be integrated within the mechanical drawings themselves. Either way, what purpose does showing BAS drawings serve, and what should they document?
Last month, I stated that BAS installations involve contractor design/build efforts. Therefore, including too much information on the drawings (e.g., controller panel layouts and wiring details) can be a waste of time since some details are determined by and vary with each BAS manufacturer’s product line. In fact, too much detail can be dangerous since: a) details that are incorrect or inappropriate for the manufacturer’s selected products can be a justification for the contractor to ignore the design in its entirety (the contractor can say “the design was sole-sourced to another manufacturer and does not apply to us.”), and b) who pays for an error in, say, a wiring detail?
There’s another reason for not showing too much detail on the drawings: CAD drawings are expensive to produce (and those labor costs would be better spent on the sequence and point list). So, what level of detail should be shown on the drawings? Experience is your best guide for determining this, though the following is some guidance about where to start.
Schematic diagrams of each unique equipment/system type controlled along with the control devices and points are a useful inclusion in the design drawings. Showing all points involved on these diagrams, not including those located on floor plans as discussed next, can also serve double duty as the project’s point list (i.e., a “point list” need not be provided in tabular form if the same information is already shown on the schematics). These diagrams are also a great aid in comprehending (and even initially writing) the sequences when a system’s complexity makes it difficult to visualize. For the same reason, however, I see no value in providing diagrams for simple systems, such as most terminal equipment (e.g., VAV boxes, FCUs, UHs) or even typical commercial VAV AHUs. Also, many mechanical designs already include what are often called “one-line” piping and airflow diagrams, which could serve double duty as the controls schematics.
Floor plans are the best way to show the quantities and locations of the various types of space sensors (i.e., temperature, humidity, pressure) and perhaps even duct/pipe pressure sensors (i.e., for control of fan/pump speed). Again, this information also represents a portion of the design’s point list. This point information can also be shown on the mechanical floor plans rather than on a separate, dedicated set of controls floor plans.
Most commercial BAS have fairly simple architectures that vary little between manufacturers. Providing an architecture diagram in these cases adds no value to the design; however, more complex projects, such as those considered mission critical, usually involve some architecture choices needed to provide BAS failure/fault immunity or even redundant control. There are many details associated with this type of BAS that are best represented on an architecture diagram. This diagram can cover issues such as how controllers are distributed amongst the various equipment/systems, how controllers might operate in a semi- or fully redundant fashion, how redundant system sensors (i.e., multiple chilled water supply pressure sensors) are connected to the BAS, the IP communications layout (i.e., the use of various subnets), how MS/TP networks are segregated and connected to the various higher-level controllers, how separate power sources are used to feed the BAS components, etc.
What are the necessary types/quantities of controls drawings for a BAS design along with the level of detail included in those drawings? It may be more than current design fees can support, but it can be a lot less than many in the industry see as necessary.