Babel Buster Network Gateways: Big Features. Small Price.
Can Too Much Be a Bad Thing?
November Issue - BAS Column
“Back to BASics” columns have focused on the fundamentals of BAS
functionality and design. In addition to the two most-important
design elements (the Sequence of Operation and Point List), a
specification is also necessary. A BAS spec is typically a
section within Division 23 (e.g., 230923), while the Sequences and
Points are usually provided elsewhere (i.e., on the drawings, in
section 230993, or some combination thereof). This column’s focus
is on the 230923 specification which includes requirements for the
submittals, products, means & methods of installation, start-up
& testing, training, warranty, etc. Which of these issues
deserve your greatest attention and how much detail is necessary?
General Considerations – BAS installation involves contractor design/build efforts. The sequence and point list mostly define the performance requirements. Therefore it can be a waste of time to go into too much detail about a system’s architecture since many of these details are determined by the manufacturer selected. Some engineers lay out the system’s physical architecture and panel designs including all of the point wiring. This may be a useful but dangerous luxury for sole-source projects (who pays for an error in any wiring/layout diagram?). However, on bid projects, this detail will be wrong for most if not all of the qualified bidders (and “wrong” information in a spec opens the door for the contractor to ignore it in its entirety). Only experience can lead you to an optimum level of specification detail, though the following is some guidance on where to start.
Submittals & Shop Drawings – This is one aspect where a high level of spec detail is generally important. Each contractor has a preferred submittal format and amount of information they like to show. For the most part, specifying the format is a waste of time since it will disrupt the contractor’s engineering process without providing any useful outcome. However, the amount of information provided should be specified sufficiently based on what you want to review. Don’t assume that you will get the following unless you specify it: floor plans showing the locations of all components (e.g., controllers and field devices), a physical architecture diagram showing how the controllers’ communications wiring is connected (along with where there are IP switches, routers, etc.), a schematic diagram of each controlled system/equipment showing point locations/names/wiring/etc., wiring/device layouts of control panels, wiring details for the various field devices, etc.
Controllers – This is one aspect where the useful level of spec detail should vary. (i.e., the “___” signifies that the word should be added). It makes sense to specify key aspects of the system’s physical architecture (e.g., “each AHU shall be controlled by no more than one B-ASC BACnet/IP controller that is dedicated to the control of that AHU only”). And to make this easier, it also makes sense to define categories of controllers (e.g., “An AHU controller shall be BACnet B-ASC BTL-listed, communicate via BACnet/IP, be a router to MS/TP VAV Controllers, have a real-time clock...” but not much more). However, don’t bother with specifying the CPU speed, RAM/ROM, point quantities for each point type, sensor input A/D resolution, etc. since these details differ greatly between manufacturers without any meaningful differences on the system performance.
Means & Methods of Installation – This is one aspect where the useful level of the spec should vary depending on…. Is it a retrofit or new construction project? A retrofit project deserves a lot of detail about what existing components can be reused (e.g., actuators/valves, sensors, wiring, enclosures, etc.) while these are moot issues for a new construction project. What is the level of the building’s criticality? The amount of detail about where/how power is obtained for the controllers and whether the power is generator or UPS-backed will increase with the criticality of the project. Is it “means” or “methods”? For example point/communication cable types can often be simply specified as “per manufacturer’s requirements”; while the installation methods of the cabling should be specified in detail (e.g., where must it be installed in raceway, is plenum-rated cable is acceptable above ceilings, can cabling in mechanical rooms be run without EMT if it is x’ above the floor, etc.).
Closing Thoughts – The necessary level of BAS specification detail: it depends! Though I will say, that my “standard” BAS spec has gotten shorter as I’ve become wiser.
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