CONTINUED PRESSURE TO DECREASE ENERGY AND MAINTENANCE COSTS and increase available space and flexibility in federal buildings has resulted in innovative and promising designs, such as raised-access flooring and underfloor-airdistribution systems, which provide supply air to occupied spaces. The use of these systems was encouraged in the 2003 edition of the Facilities Standards for the Public Buildings Service: PBS P100, or P100, published by the U.S. General Services Administration, Washington, D.C., for its new construction projects. GSA now has more than 8 million square feet (743200 m2) of UFAD in 14 buildings and is evaluating the systems’ performance.
The popularity of UFAD is understandable. Using the space under the raised-access floor as a pressurized plenum rather than relying on overhead or underfloor ductwork is attractive because of the potentials for lower first costs, easier coordination between HVAC and other systems, and minimized labor to modify supply air distribution when changes are made to the occupied space.
However, UFAD systems with pressurized plenums have not performed as expected in recently completed federal buildings. Therefore, this configuration for UFAD has come under scrutiny by GSA. Lessons learned from the successes and failures of these innovations are the basis for new GSA design standards and criteria. As the green-building industry moves forward, those interested in specifying UFAD systems should learn how to design and install these systems to assure building owners UFAD systems perform in accordance with specified design intent.
LESSONS LEARNED / As part of the GSA’s HVAC Excellence Initiative, the HVAC Excellence Quality Assurance Review process was developed to ensure designs for new or renovated buildings are in compliance with P100. Lessons learned from the review process illustrate that UFAD systems in some GSA facilities are creating several issues, including the following:
Plenum air leakage is an architectural design and general construction issue. It is a mechanical engineering concern to the extent that the capacity of the air-handling unit must be adequately sized to compensate for the leakage at design load, and controls must be designed to compensate for the air leakage at partial loads. Category 1 and 2 plenum air leakage tests at design static pressures conducted in six federal buildings and courthouses have ranged from 30 to 200 percent of the design airflow rates. Some of the consequences of such air leakage include complaints of being cold around the feet and legs from Category 2 air leakage during partial-load conditions, or when the thermostatic setpoint was satisfied; energy wasted because AHUs were required to run at higher-than-expected airflow rates, static pressures and longer periods to compensate for Category 1 and 2 air leakage; operations and maintenance difficulties in accessing equipment in the plenums; the need to reseal Category 1 leakage pathways because of changes in electrical, communications and plumbing penetrations; and IAQ issues and housekeeping difficulties caused by accumulation of inert and biological matter and pest infestation in the raised- floor plenum.
The thermal mass of the slab and plenum walls is a significant issue for energy management and control. Observations revealed that longer-than-expected operational periods of the HVAC systems were required to maintain plenum temperatures. Heat and moisture transmission and condensation in the plenum also are issues because gradients across the plenums resulted in non-uniform temperatures in the occupied spaces, and surfaces within the plenums were more likely to support condensation.
SOLUTIONS / Methods to deliver and maintain safe and functional pressurized plenums must be developed by a coordinated effort among designers, building owners/managers, product manufacturers, contractors, building-code officials and standards-writing organizations.
Construction of an airtight plenum requires strict coordination of 10 to 12 trades and special construction techniques that have not yet been developed for concrete, masonry, drywall, millwork, sealant and joint specialists, raised-access floor installers, carpenters, sheet metal, plumbing, electrical, communications, etc.
Because predictions of air leakage are unreliable, GSA is requiring testing at this time. The organization also is investigating use of fully ducted UFAD systems that eliminate the problems associated with plenum air leakage. The lessons learned are being addressed by the Atlanta-based American Society of Heating, Refrigerating, Air-Conditioning Engineers Inc.’s newly formed Technical Resource Group 7 for Underfloor Air Distribution, which intends to provide updated guidance about the design of UFAD systems. For more information about this group, contact ASHRAE at (404) 636-8400.
CATEGORY 1 AIR LEAKAGE General construction leaks from the plenum into other building cavities. An example would be leakage around and in annular spaces in conduit. CATEGORY 2 AIR LEAKAGE Product leakage through the raised-access flooring into conditioned spaces, such as leaks of conditioned air from the plenum through floor-panel seams and edge closures, electric-power connections and outlet service units, as well as air diffusers that do not close tightly.
> > JAMES E. WOODS is executive director of The Building Diagnostics Research Institute Inc., Chevy Chase, Md., and chair of the ASHRAE Technical Research Group 7 for Underfloor Air Distribution. VIJAY GUPTA is chief mechanical engineer, Office of the Chief Architect, U.S. General Services Administration, Washington, D.C., and a member of TRG7-UFAD. Woods can be reached at email@example.com or (301) 951-5951, and Gupta is available at vijay.gupta @gsa.gov or (202) 501-0628.