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VAV Reheat Versus Active Chilled Beams & DOAS

©2013 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 55, no. 5, May 2013.

By Jeff Stein, P.E., Member ASHRAE; and Steven T. Taylor, P.E., Fellow ASHRAE

About the Authors
Jeff Stein, P.E., and Steven T. Taylor, P.E., are principals of Taylor Engineering in Alameda, Calif. Stein is a consultant to SSPC 90.1 and a member of SPCs 155P and 195. Taylor is a member and former chair of SSPC 90.1 and is vice chair of TC 4.3, Ventilation Requirements and Infiltration.

Several recent articles claim 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. 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 provided a Design Development (DD) level design for a real office building currently in design, the UC Davis Medical Center Graduate Studies Building (GSB) in Davis, Calif. One firm designed an ACB+DOAS system, another firm designed a VAVR system, and the third designed a hybrid combination of these two systems. A fourth engineering firm 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 the lowest first costs and the lowest energy costs of the three systems. The analysis showed that many of the 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 also may apply more broadly.


Genesis of Competition

The UC Davis Medical Center Graduate Studies Building (GSB) will be a 56,500 ft2 (5249 m2) office building. The space program is fairly evenly split between private offices, open offices and classroom/conference rooms. Chilled water and hot water will be provided by the campus central plant.

When UC Davis first decided to build the GSB, it started with a traditional plans/specifications approach with Firm A as the engineer-of-record. Firm A, which has designed more than 1 million ft2 (92 900 m2) of chilled beam buildings, chose an ACB+DOAS design. In early 2012, when the design was in the design development (DD) stage, the owner decided to switch to a design/build approach, and Firm A’s design became the bridging documents.

One of the design/build teams bidding on the project proposed a VAVR system (designed by Firm B, the authors’ firm) and also carried the design through about the DD level in its bid. Another bidder proposed a hybrid chilled beam + VAV reheat system, designed by Firm C. The three designs included equipment schedules, detailed equipment layouts and zoning plans. The team with the hybrid design was awarded the job, but the project has been placed on hold since.

After UC Davis selected a design/build team, the ASHRAE Golden Gate chapter decided to use this building for a competition between chilled beams and VAV reheat since it had already been designed with both systems and with a hybrid combination of the two. All three firms were eager to participate. The results of the competition were presented at a seminar sponsored by the ASHRAE Golden Gate Chapter at the Pacific Gas & Electric Energy Center in San Francisco on Oct. 17, 2012.


Active Chilled Beam Design

The ACB+DOAS design consists of a 100% outside air, constant volume air handler serving two supply risers (Figure 1). The air handler has a chilled water coil with a bypass damper and a hot water coil. The air handler has a variable speed drive primarily for reducing fan speed when the bypass damper is open. The air handler provides the primary air to the active chilled beams. The average DOAS primary airflow rate is about 0.6 cfm/ft2 (0.28 L/[s•m2]), which is considerably higher than the minimum ventilation in low occupant density spaces, like offices, but close to the minimum ventilation in higher density spaces such as conference rooms and classrooms. A DOAS flow rate higher than the minimum ventilation in low density spaces is often needed to meet the space loads as the capacity of the chilled beams is a function of the primary airflow rates. In densely occupied spaces, it also ensures space dew point does not rise above the surface temperature of the chilled beams, possibly causing condensation. It also improves the indoor air quality compared to code minimum ventilation.


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