CONTROLS SYSTEM DESIGN

I, too, am delighted to see some emerging efforts that show great promise in providing better design tools to engineers and other BAS specifiers. The question is will these efforts be too little, too late?

If we look back on the pre-DDC days of 1980-ish and earlier, the fact is that control systems were relatively simple — i.e., there was only so much you could do with pneumatic and electric controls before they would fail under the weight of excessive complexity. HVAC designs were also much simpler since none of the myriad of energy conservation-driven codes, standards, and incentive programs (along with the associated technical advances) were yet in place.

In fact, HVAC system choices and variations in the pre-DDC days were so limited that it was possible to develop a very reasonable and repeatable set of standard sequences of operation. Better still these “standards” were well understood and documented (along with the controls designs required) by the major controls manufacturers. This meant that controls “design” was a fairly straightforward effort. It could even involve substantial assistance by a local TC contractor without fear that this would give that contractor an unfair bid-day advantage (due to equivalency of most pneumatic/electric controls).

Today, we enjoy none of the above benefits from the “good old days.” Don’t get me wrong, I don’t long for those days just because the design efforts were substantially less. In fact, just the opposite. The great variety of today’s HVAC design choices (and code/standard constraints) have allowed (forced?) us to greatly innovate how BAS are applied to our projects. However, framed against this backdrop of innovation is the sad reality that design fees have fallen. Is it any wonder that the effort put towards today’s controls designs cannot even rise to the level afforded during the “good old days” (maybe they were really better?).

The great variety of today’s HVAC design choices (and code/standard constraints) have allowed (forced?) us to greatly innovate how BAS are applied to our projects. However, framed against this backdrop of innovation is the sad reality that design fees have fallen.

So all of this puts a lot of pressure on initiatives to provide much better BAS design guidance/assistance. Fortunately, a closer look at the not-yet-official ASHRAE Guideline 36P “High Performance Sequences of Operation for HVAC Systems” indicates that it might be just what is needed.

The stated purpose of Guideline 36P (from “Part 1 – Purpose”) is “…to provide uniform sequences of operation…” Note the word “uniform,” which indicates that this is not intending to be merely a guideline but a reference manual of standardized sequences. Further, “2.1 Sequences of Operation” says “Lists of hard-wired points and control diagrams are included.” Wow, this is beginning to sound like the good old days!

Further inspection of the Guideline 36P document reveals how seriously it is attempting to fulfill the stated purpose. First off, the first 20+ pages of “Part 5 – Sequences of Operation” cover detailed definitions of the various elements that go into the sequences. These include such things as a “control loop;” how “trim and respond” is to be used for resetting setpoints (e.g., duct static based on VAV box demand); how to stage and rotate equipment (e.g., multiple chillers); and much/much more. Finally, the specific sequences provided are way more prescriptively detailed than anything even the most sleep-deprived engineer could ever justify developing for most projects.

If successful, the sequences and point database described in Guideline 36P could be programmed into BAS controller firmware. In other words, these would be canned sequences in the same manner as many of today’s VAV box controllers. This would allow more complex equipment/systems (e.g., VAV AHUs, central plants) to benefit from a far simpler installation, setup, and testing process (not to mention the on-going maintenance).

For those who haven’t looked at this document, please do. It is readily available at http://gpc36.savemyenergy.com/public-files/. It will be very interesting to see whether it can evolve into a tool that is embraced as a “standard” by the industry so that controls design can once again be as simple as it was in the “good old days.” ES