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The Coming Evolution of BAS Design
A good BAS design starts
with a well thought-out
Paul Ehrlich & Ira Goldschmidt
We all would agree that the commercial building HVAC industry continues to become more complex, both for designers and contractors. With the need to meet ever-stringent energy and IAQ standards, achieve LEED certification, adding intelligent building capabilities, etc., along the always-shrinking design fees and building space available for systems it’s a wonder that new buildings work as well as they do. Experience shows that the major issues of concern to a design engineer are that the equipment must fit and it can’t be undersized; thereafter, there is usually some leeway concerning the use of “performance-based” design criteria so that the project’s design can be completed for the fee available.
BAS’s are no different than the industry in general - the technology has continually become more complex which has led to a more challenging design process. Because of this challenge there are appears to be two philosophies towards design that are being used or considered:
· BAS Design should continue to be the responsibility of the design engineer, though a performance-based design is sufficient.
· BAS Design should be relegated to the temperature controls contractor. This includes development of the sequence of operation and a point list.
The first choice is clearly that which is being used on the majority of current projects. There are a lot of good reasons for this especially since it is pretty much “the way it has always been done”. More importantly the engineer has the most detailed knowledge of the systems’ intended operation and functional requirements, knowledge that is key to the BAS design process. However, the growing acceptance of a more performance-based design is one of the main sources for the controls issues that result from many of today’s projects. Why?
With the current standard practice of using open protocols to connect the BAS to chillers, RTU’s, VFD’s, etc. there is no one generic “performance” design that can adequately cover the design detail variations between the various equipment manufacturers involved. Unfortunately, a prescriptive BAS design usually takes more time or, at least, requires specialized controls knowledge on the part of the engineer. Unfortunately, the training needed for this knowledge is difficult to come by and, when available, is generally not being provided. A traditional mechanical or architectural engineering degree program provides very little training on the intricacies of modern commercial controls. Secondly, the multi-disciplinary subjects involved (control theory is an electrical engineering subject, open protocols knowledge is grounded in data communications, etc.). The best way to gain this knowledge is via the actual hands-on training that a controls technician receives, but this is unfortunately a career path that most degreed engineers do not normally take.
So what about the second approach (which is often discussed but rarely used)? There are theoretically some advantages to this approach IF the temperature controls contractor is selected as a “design-assist” partner early in the design process. Of course, this unusual step can only be achieved via a paradigm shift in how GC’s/MC’s do business. However, we believe that this approach would help to develop the “Systems Integrator” function within the construction industry (a concept which our industry has talking about for years without much result). Therefore we encourage owners, engineers and contractors to take the bold step of trying this approach so that the short-comings discussed above can be addressed. Simply put, the involvement of a knowledgeable controls contractor early in the project would reduce the need for generic product specifications and would allow the design to be based on the actual product/equipment capabilities to be provided.
Barring any change to how the industry provides BAS’s we need to stress something that this column has stated in the past. A good BAS design starts with a well thought-out sequence of operation and point list (one based on the actual equipment-provided controls expected). With that in hand maybe the design philosophy chosen isn’t all that important after all!
About the Authors
and Ira first worked together on a series of ASHRAE projects including the
BACnet committee and Guideline 13 – Specifying DDC Controls. The formation of
Building Intelligence Group provided them the ability to work together
professionally providing assistance to owners with the planning, design and
development of Intelligent Building Systems. Building Intelligence Group
provides services for clients worldwide including leading Universities,
Corporations, and Developers. More information can be found at
www.buildingintelligencegroup.com We also invite you to contact us
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