ASHRAE Journal - June 2010 - 14

Multiple-Zone Systems in 62.1
“Dynamic Reset for Multiple-Zone Systems” by Dennis Stanke, Fellow ASHRAE, in the March issue presents clearly the challenges and possibilities of Standard 62.1-2007. As practicing engineers, we have always appreciated in ASHRAE the basic and commonsense approach to practical problems, and we were fully behind their review of air quality issues that led to the development of the later version of the standard. What is more difficult to follow and explain to our clients are the merits of the more advanced and scientific, but more complicated and abstract calculations involved in the multiple-zone approach of the current version of Standard 62.1. This article, as with the others previously published in the Journal, was good at presenting, in detail, the multiple-zone calculation process. At the end of the article, as well as for Tables 2, 4 and 6, the outdoor air intake flow was established at: • 26.2 cfm/person, of design diversity population; • 19.2 cfm/person, of design zones peak population; • 18.9 cfm/person at 80% design airflow, of design system population; and • 22.6 cfm/person at 80% of design population. The total outdoor air intake flow was reduced by 45%, from 4,300 to 2,960 cfm, with the development of these analyses. All of these can be deemed compliant with the standard. This brings me back to my problems with my clients who easily grasped the concept of 20 cfm/person but are somehow lost when I give them the new multiple-zone analysis. Again, at the end of the day, after all the drinks are done and the glasses are empty, I will be glad to note how these sophisticated numbers approach our old standard of 20 cfm/person, close enough by engineering practice. For your information, the National Building Code of Canada in its latest version chose to retain the old standard of 20 cfm/person for offices for outdoor air supply, as per Standard 62-2001. This will certainly facilitate enforcement by regulating authorities, and dealings we engineers have to carry with these colleagues. Christian R. Roy, ing., Member ASHRAE, Montréal, QC, Canada ately large (or inappropriately small) rates that historically resulted when using combined rates (such as 20 cfm/ person) at the zone level. Standard 62.1 also requires accounting for system ventilation efficiency in multiple-zone systems to avoid inappropriately high (or inappropriately low) outdoor air intake rates that historically resulted from simply adding zone outdoor airflow rates without taking credit for unused outdoor air in the recirculated airstream, or accounting for the high fraction of outdoor air needed in the critical zone. While an effective intake flow of 20 cfm/person may seem “right” for this office example, it would not seem “right” in systems with other occupancy categories (such as theatres requiring a combined rate of only 5 cfm/person). Furthermore, the unnecessarily high combined zone rates needed to “simplify” multiple-zone system calculations would raise the total intake airflow required for single-zone and 100% outdoor-air systems to unacceptably high levels. Although both Standard 62 and the IMC have required very similar calculations for multiple-zone systems since the early 1990s, it’s unfortunate that some of your clients don’t accept them. They apparently want to be able to “do the math” whether it’s simple to grasp (intake airflow equals 20 cfm/person times design population, for instance) or more difficult to grasp (zone outdoor airflow equals Rp cfm/person plus Ra cfm/ft2 and outdoor air intake flow equals outdoor air used in all zones divided by system ventilation efficiency). It might be to your advantage to simply show your clients the answer rather than the calculations required to find the answer. You might also consider submitting a change proposal to ASHRAE Standing Standards Project Committee 62.1, to offer a simplified calculation approach. Once again, thank you for your interest in this topic. Dennis Stanke, Fellow ASHRAE, La Crosse, Wis.
June 2010

The Author Responds
Thank you for your interest in this article. Standard 62.1 separates the outdoor airflow rate needed for peoplerelated sources from that needed for building-related sources. This approach was adopted to avoid the inappropri-

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ASHRAE Journal - June 2010

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