VAV Reheat Versus Active Chilled Beams and DOAS
There have been a number of articles recently claiming that Dedicated Outdoor Air Systems (DOAS) plus active chilled beam (ACB) systems are superior to Variable Air Volume Reheat (VAVR) systems on energy efficiency, first cost, air quality, etc.[see references 1-4]. The ASHRAE Golden Gate Chapter recently decided to hold a head-to-head competition to put these claims to the test. Three mechanical engineering firms with offices in the Bay Area each provided a Design Development (DD) level design for a real office building currently in design, the U.C. Davis Medical Center Graduate Studies Building (GSB) in Davis, California. One firm designed an ACB+DOAS system, another firm designed a VAVR system, and the third firm designed a hybrid combination of these two systems. A fourth engineering firm then simulated each of the three designs using the EnergyPlus energy simulation program. Finally, a major mechanical contractor provided a detailed HVAC construction cost estimate for each design. The VAV reheat system had both the lowest first costs and the lowest energy costs of the three systems. The analysis also showed that many of the other supposed advantages of ACB+DOAS relative to VAVR, such as improved indoor air quality and a lower floor/floor height, also turned out to be largely overstated. Note that the results of this analysis are only strictly applicable to these three designs and this building and climate, but the conclusions may also apply more broadly.