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.