Project Title:

Performance of Underfloor Air Supply Plenums

Providing specialized resources and knowledge required for proper design and operation of underfloor plenums.

Status (updated 6/18/2009): Ongoing

Funding sources: CBE Industry Consortium, Research Grants

Project Objective

Develop design and specification guidelines to determine optimal system configuration and operating strategies for underfloor air distribution (UFAD) plenums. Include findings in UFAD technology transfer through industry publications, in UFAD technology website, and in CBE reports. Conduct full scale experiments to develop a validated version of the energy simulation tool EnergyPlus capable of modeling UFAD systems.

Significance to Industry

CBE has learned from field studies and industry feedback that incorrect design, specification and/or construction of UFAD plenums may be contributing to poorly performing buildings. Such projects do not realize the full benefits possible with UFAD systems, and slow wider adoption of the technology. Industry practitioners require specialized knowledge to avoid potential problems related to thermal decay, leakage, and thermal comfort.

Research Approach

In this research area a number of full-scale field experiments have been conducted to investigate the airflow and thermal performance of underfloor air supply plenums. CBE maintains a full-scale UFAD test facility, consisting of a 22-foot by 74-foot by 1-foot high plenum. The test bed includes complete HVAC and data acquisition systems. CBE also utilizes computational fluid dynamics modeling to study plenum performance, including air flow and thermal transfer.

A combination of empirical full-scale experiments and computer simulations have been used to study: (1) thermal storage in the concrete slab; (2) heat transfer rate to the underfloor air flow; (3) air temperature variations across the plenum; and (4) heat transfer through the access floor. Tests have studied the minimum effective plenum height that allowed acceptable air flow performance in a pressurized underfloor air supply plenum, and the effect on thermal transfer in underfloor plenums in multi-floor buildings with stratified air temperatures. A simplified plenum model was also been developed to predict underfloor plenum thermal performance using the EnergyPlus energy simulation program.

Additional testing in the plenum test facility is investigating leakage issues, plenum inlet conditions, control issues, different diffusers and fan-powered underfloor mixing boxes, and practical construction and fabrication issues.

The research team for the UFAD and plenum projects has published findings from this research extensively in academic and trade journals, and has been involved in numerous conferences, workshops, and symposia.

Publications and Reports

Bauman, F., T. Webster, and D. Dickerhoff, 2008. Air Leakage Test Report EPA Region 8 Headquarters, Denver, Colorado. CBE Summary Report, August.

Bauman, F., T. Webster, and H. Jin, 2006. Design Guidelines for Underfloor Plenums. HPAC Engineering, July. (external link)

Bauman, F., T. Webster and P. Pecora, 1999. How Low Can You Go? Air Flow Performance of Low-Height Underfloor Plenums. CBE Summary Report, October. (PDF 189K)

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Bauman, F. 2005. Slide presentation from BetterBricks/ASHRAE Workshop. Seattle WA and Portland OR, September 7-8, 2005. (PDF 3MB)

CBE UFAD Test Facility

CBE's full-scale underfloor plenum test bed.

CBE UFAD Test Floor

Temperature sensors imbedded in the concrete slab for a study of thermal transfer.

CFD Model of UFAD Plenum

Plenum laboratory testing is complemented with modeling using computational fluid dymanics (CFD) .