©2019 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 61, no. 4, April 2019.
About the Authors
Gwelen Paliaga, P.E., is a technical director with TRC within the Research and Technology Commercialization group, Oakland, Calif. Tyler Hoyt is a software engineer at Comfy Inc., Oakland, Calif. Hui Zhang, Ph.D., is a researcher and Edward Arens, Ph.D., is a professor of the Graduate School and director at the Center for the Built Environment, at the University of California, Berkeley, Calif.
A large percentage of commercial buildings in North America use variable air volume (VAV) systems with reheat, and this system type is also common around the world. Summertime overcooling is widespread in such buildings and has received considerable media attention over the past few years. ASHRAE Research Project RP-1515, reported in this article, shows that much of today’s overcooling originates in unsubstantiated engineering assumptions about the performance of VAV boxes and diffusers at low-flow setpoints. These assumptions are that low flows will cause diffusers to dump cooled air and create drafts around occupants, ventilation air will be poorly mixed, and VAV airflow control will become unstable or inaccurate. Together, they have resulted in VAV minimums being commonly set at 20% to 50% of maximum. ASHRAE RP-1515 and other recent research have shown each of these assumptions to be unwarranted, and that far lower minimums are desirable.
In RP-1515, buildings operated on corrected assumptions were found to reduce their pre-existing cold complaints by half while also saving energy. Reducing VAV box minimum airflow setpoints to ventilation minimum flow rates, often around 10% of maximum, reduced total HVAC energy by 10% to 30%, which is remarkable for an inexpensive controls setpoint change that properly maintains outside air ventilation. There were no draft discomfort complaints during low flows, and the preexisting rate of occupant cold discomfort was cut in half. The new control sequences are applicable to all new and many existing VAV buildings.
Background
VAV box minimum airflow setpoints have tremendous energy implications. Simulations have suggested that lowering the minimum airflow setpoint to the levels needed for outside air ventilation (~10% of maximum flow) would reduce a conventional building’s HVAC energy by 10% to 30% [Hoyt et al. 2014, Taylor et al. 2012]. However, conventional practice (minimums set between 20% to 50% of maximum flow) has been firmly based on long-standing concerns among designers and manufacturers about the indoor environmental quality under low minimums. In other words, will there be a downside to the indoor environmental quality under low minimum operation?
VAV minimum flow setpoints have been traditionally maintained at the higher levels because of three concerns held by practitioners and manufacturers:
- VAV boxes might be unable to sense or control low flows;
- Poor air quality might result from a combination of poor control and insufficient diffuser mixing; and
- Low flows might cause the occupants to perceive draft discomfort from insufficient mixing of diffuser discharge air.
To address these concerns, diffuser manufacturers and ASHRAE Handbook—Fundamentals have for many years suggested that minimum VAV airflows be limited to 30% to 50% of design airflow. However, there was little research supporting the suggestions.
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