Home > Articles By Issue > Energy & Environment > Article July 2003

Walking On Air

By Joseph M. Halza, York International Corporation

Underfloor air distribution systems (UFADS) were introduced in the 1950s to address the cooling needs of computer rooms. Large mainframe computers required a raised-floor system capable of delivering massive amounts of cooling beneath the equipment while providing easy access to electrical services. Floor panels were installed on pedestals above the concrete slab to create an underfloor space.

As technology became more sophisticated, computers were distributed throughout the workplace. Raised floor construction continued to evolve, driven by the need to manage networks of power, voice, and data cabling.

Underfloor air distribution technology is experiencing a similar resurgence, due to the development of underfloor variable air volume (VAV) technology that permits the precise comfort control required in today's workplaces.

What UFADS Do

UFADS take advantage of the plenum between the structural concrete slab and the access floor to move conditioned air throughout a facility. HVAC can be situated in this plenum, eliminating the need for supply ductwork and allowing the installation of diffusers in the raised floor grid. Air is exhausted from the room through return grills located near the ceiling.

Unlike traditional overhead systems that attempt to condition all the air in a given space, current UFADS carefully target the occupied zone (OZ) defined by ASHRAE Standard 55-1992 as the vertical space from 4' to 67' above the floor. As a result, clean, conditioned air is delivered to the occupants first.

After induced mixing in the lower portion of the OZ, the air picks up heat and pollutants. Natural convection carries the air to the ceiling where it is discharged. This "convection enhanced ventilation" provides occupants with cleaner air, because it does not pass through pollutants that congregate at the ceiling.

Energy Benefits

Today, UFADS are only being specified in approximately 10% of new construction projects. However, users are finding that these systems can deliver clean, conditioned air while affording flexibility to facility managers and energy savings to building owners. Compared to overhead air distribution systems, UFADS save energy because they can deliver slightly warmer air at lower static pressure. Consider the following aspects of energy savings:

UFADS can deliver 60¡F to 65¡F air versus a conventional system's 55¡F air. This can result in a reduction in mechanical cooling and increased use of outside air for "free cooling." While the amount of savings will vary with each application, it can represent significant dollars in both operating and life cycle cost.

Further, UFADS are designed with a supply air static pressure of 0.05"w.g.-compared to a typical overhead system at 1.5"w.g. to 2.0"w.g. (or inch of water gauge, another common name for the inch of water column. For a more complete definition of w.g., see "How Many? A Dictionary of Units of Measurement," by Russ Rowlett and the University of North Carolina at Chapel Hill, www.unc.edu/~rowlett/units/dictI.html.) This means fan energy usage can be reduced. This is significant, because fan energy accounts for approximately 40% of total HVAC system energy.

Some UFADS can achieve additional energy savings through a perimeter system design that limits the operation of fan powered terminals to the winter months only. Add to these savings a faster depreciation schedule (because most of the UFADS components are considered equipment rather than part of the building), and life cycle costs drop even more.

For buildings being designed with Leadership in Energy and Environmental Design (LEED) certification as a goal, a VAV UFADS can contribute to as much as 60% of the LEED points required for building certification. Energy savings and IAQ levels associated with UFADS enhance the potential for certification.

Construction Costs

The cost to install an underfloor system is typically less than the installed cost of a traditional overhead system.

The elimination of supply air ductwork reduces first costs and construction time due to a reduction in field labor hours. Scaffolding and ladders are no longer required, as installation occurs at floor level in a matter of minutes per terminal unit.

UFADS also can lead to savings in building construction. Slab-to-slab height can be reduced by as much as 1' per story, resulting in one "free" story for every 12 floors constructed.

Exterior fa�ade costs, too, are reduced, sometimes as much as 10% per story. Inside, interior design is not limited by ductwork, making it easier to include day lighting and indirect lighting in the design.

UFADS enable facility managers to make field modifications quickly and inexpensively without disrupting workflow, because no ductwork modification is required. Even after the building is occupied, access panels can easily be reconfigured.

This offers comfort as well as flexibility in the workplace and allows facility managers to respond quickly and economically to churn or renovation projects. Office space can be reorganized and the accompanying HVAC system reconfigured without reconstruction, interference with other mechanical systems, or expensive outside labor.

Building Integrity

Because a building's floor slab, ceiling, walls, and column enclosures are the structure of the supply and return plenums, they must be sealed to prevent air leakage. Leakage can actually offset some of the diffuser capacity and impact the ventilation requirements.

This means specifying and detailing procedures during construction as well as operation of the UFADS that will guarantee leak tight underfloor plenums at the curtain wall, furred in columns, penetrations for pipes, ducts, wires and cabling, and stud walls that penetrate the raised floor. Without attention to building integrity, the UFADS may not perform as designed, and occupant comfort could be compromised.

Pressure dependent diffusers with a minimum ventilation stop should ordinarily be coupled with proper CO2 monitoring to assure facility managers that occupants receive an adequate supply of fresh air. As a secondary measure, they can review tested leakage rates for floor panel/carpet combinations, diffusers, and power, voice, and data boxes. It is also a good idea to coordinate static pressure drops in supply and return shafts with fan performance of the air handling units. This will help to avoid pressurization problems and loss of economizer capability.

Design Basics

Return air grills should be placed close to the perimeter wall. Locate diffusers in functional and aesthetically pleasing layouts, making sure airflow patterns match drawings and avoiding center traffic areas, partition lines, and furniture.

A cooling load calculation should be the basis for the terminal density in a particular space. However, with today's state-of-the-art systems, one terminal unit and grill per 200 to 250 square feet usually meets the cooling requirements of a typical office setting. During construction, the slab, access floor, and terminal boxes must be cleaned and vacuumed. Facility managers should also train maintenance staff to run the system and equip them with the appropriate tools to test thermostat operation; a calibrated flow hood can verify underfloor pressure and airflow delivery.

Making People Comfortable

As with any significant change in a building, it is a good idea to provide building occupants with an understanding of the UFADS. Indicate the location of thermostats and sensors for employee areas (or "zones"), explain their functions, and demonstrate how to use them.

Some manufacturers offer occupant training materials in CD and video formats that explain the operation of the system. When the system operates better, this can help create a higher level of employee satisfaction.

The application possibilities of UFADS continue to grow. Where plans for raised flooring are in place to address wire management and flexibility, the addition of a UFADS may enable facility managers to address the needs of constantly changing work environments easily and economically. Clearly, occupant comfort and improved IAQ will bring additional long term benefits for managers, owners, and everyone else involved in the facility.

Halza is a product marketing manager for York, PA-based York International Corp., where he has been employed for 15 years in various engineering and marketing positions. He holds a BSME from the University of Pittsburgh and has been an active member of ASHRAE for over 16 years.

Have you considered switching to UFADS? Share your thoughts by sending an e-mail to schwartz@groupc.com.