Initial Evaluation of Displacement Ventilation and Dedicated Outdoor Air Systems for U.S. Commercial Buildings

Published

July 1, 2005

Author(s)

Steven J. Emmerich, Timothy P. McDowell

Abstract

Conventional heating, ventilation and air-conditioning (HVAC) systems in commercial buildings meet both outdoor air ventilation and space conditioning requirements using air distribution approaches that provide supply air that is a mixture of outdoor air and recirculated air with the goal of achieving a well-mixed space. More recently, advanced ventilation approaches have been proposed, and in some cases installed, that separate the outdoor air ventilation and space conditioning functions (dedicated outdoor air systems or DOAS) or distribute air to provide thermal stratification (displacement ventilation or DV). NIST has completed an initial evaluation of the potential benefits and limitations of DV or DOAS in lieu of the conventional mixing ventilation (MV) approach common in U.S. commercial buildings.The first task in this effort was a compilation of information on DV and DOAS systems. This included a review of the scientific literature from sources including ASHRAE, the Air Infiltration and Ventilation Centre, and Indoor Air and other international conferences. There is a large body of literature on the design and performance of DV systems including two recent design guides one published in the U.S. and the other from Europe. As reported in the literature and summarized in the design guides, DV systems have the potential to improve indoor air quality (IAQ) in commercial buildings while also reducing cooling energy use relative to traditional MV systems. However, a close examination of the literature also reveals the need for caution in considering these benefits and care in applying DV systems. Chief among the concerns is that the IAQ improvement is only for some types of contaminants from some types of sources, cooling energy reductions may be offset by heating or fan energy increases, and thermal comfort is not easily achieved and maintained. Despite these concerns, DV systems have promise and are worth pursuing but they have not yet been proven effective in the U.S. applications.The literature on DOAS contains far fewer detailed studies. Advantages of DOAS cited include improved humidity control, assured delivery of proper ventilation airflow quantities, and reduced energy use. Although one field study of DOAS application in Florida schools supports some of the claimed advantages, the energy and humidity control performance of these systems has not been adequately studied in real buildings.Since the literature contains more analyses of the performance of DV systems than DOAS, the second task of this effort was a simulation study of the potential energy impacts of DOAS systems in a small U.S. office building. Simulations were performed with a combined airflow-building energy modeling tool linking TRNSYS and CONTAM. Based on the simulation results, the DOAS showed promise in reducing energy consumption relative to the baseline system in all climates studied. Also, a more complex DOAS did not show significant improvement over a simple DOAS consisting of only a preheat coil and enthalpy wheel.A key outcome of this effort is recommendations for future research and technology transfer efforts based on this initial evaluation. These recommendations include further detailed study of humidity control and other performance aspects of DV systems, field studies of DV and DOAS installations in U.S. commercial buildings, and extension of the U.S. Environmental Protection Agency s modeling tools to include DOAS capability.

Citation

NIST Interagency/Internal Report (NISTIR) - 7244

Report Number

7244

NIST Pub Series

NIST Interagency/Internal Report (NISTIR)

Pub Type

NIST Pubs

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Keywords

air distribution, dedicated outdoor air systems, displacement ventilation, energy efficiency, indoor air quallity, modeling, outdoor air, ventilation

Citation

Emmerich, S. and McDowell, T. (2005), Initial Evaluation of Displacement Ventilation and Dedicated Outdoor Air Systems for U.S. Commercial Buildings, NIST Interagency/Internal Report (NISTIR), National Institute of Standards and Technology, Gaithersburg, MD, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=860994 (Accessed April 24, 2024)