Making a Case for Underfloor Air Distribution Systems
June 9, 2008
ST. LOUIS - For many solid reasons, underfloor air distribution (UAD) systems are gaining popularity. In 1995, less than 3 percent of new office buildings had raised floors, with UAD being a fringe element. By 1999, 8 percent of new offices used raised floors, 20-25 percent of these with UAD systems. By 2002, 12-15 percent have raised floors, with approximately 40 percent of these having UAD systems in place.
“It has so many things going for it,” assured Rick Tinucci, senior vice president of Bick Group, which has been providing one-source solutions for mission-critical facilities in the United States since 1964.
In fact, when it came time to renovate a one-time, old, 49,183-square-foot printing press building into its bright, state-of-the-art, LEED Gold-certified headquarters in St. Louis, Bick Group didn’t think twice about installing such a system.
“It really was a no-brainer,” said Frank Bick, executive vice president.
The company saw its value since day one. After all, it plans, designs, constructs, maintains, and operates facilities such as data centers, computer rooms, broadband telecommunications facilities, and other such mission-critical facilities. And, for the most part, data centers and computer rooms use raised floors and UAD systems.
“Because Bick Group is regularly engaged in projects throughout the facility’s life cycle, we gain a unique understanding of what works and what doesn’t work,” said Tinucci. “This works.”
ADVANTAGES DOWN UNDERIn his estimation, owners of office buildings should not think twice about incorporating UAD systems. “No matter the intention, be it a green facility or not, cost containment in building design and construction is always an issue,” said Tinucci.
“And with the rising costs of construction and energy, building owners and developers are seeking innovative methods to provide an infrastructure capable of accommodating up-to-date technology, but still offering the ability to contain costs. One such system growing in popularity is the use of underfloor air distribution.”
The advantages are plentiful, he said, including:
• It takes advantage of air’s natural tendency to rise as it warms.
• Air distributed from the floor is introduced at a lower velocity and requires less heating and cooling than air from overhead systems, which reduces energy consumption and operating costs.
• There’s a lower installation cost associated with no overhead ductwork and very little underfloor ductwork.
• Distribution components (diffusers, underfloor variable-air volume boxes) can be easily reconfigured and reused, reducing costs associated with churn.
• Incoming fresh air is not mixing with the warm, spent air at the ceiling, so occupants get first benefit of the air.
• Employees can adjust diffusers in their work areas to their individual comfort. And,
• Reduced absenteeism and productivity gains associated with better IAQ means substantial cost savings potential.
“Somewhat controversial, this technology was introduced in the 1950s in spaces with high heat loads such as computer rooms and control centers,” explained Tinucci. “It was then subsequently adopted in office buildings in the 1970s. Since then, UAD has received considerable acceptance internationally, especially in Japan and throughout Europe.”
Bick Group is doing its part in spreading the good news regarding UAD. It recently invited several members of the media to tour its St. Louis facility. Representatives from Tate Access Floors, which co-hosted the gathering, were also on hand.
“Over 450 million square feet of Tate access floor systems have been installed worldwide,” said Scott Alwine, marketing manager, Tate Access Floors (Jessup, Md.). “We currently have over 70 percent of the market share.”
NOT AS COSTLY AS YOU THINKWhile raised floors are not a new concept, the move to the digital environment is making UAD an increasingly popular concept. Unlike data centers that use raised floors as high as three feet, UAD raised floors range from 10 inches to 18 inches. The raised floor plenum also is used for power and data cabling.
The biggest disadvantage of the technology is the lack of familiarity that many designers, owners, tenants, and builders have with it. Buildings are a major long-term investment, and no one wants to be involved in an experiment. It’s the proverbial Catch-22.
A major misperception about UAD centers is on the expense. There is a misconception that buildings with UAD are more expensive to build, maintain, and run than those buildings using conventional overhead cooling.
After all, why would a cost-conscious owner or developer opt to pay the premium for an access floor? Answer: To reduce the expenses incurred by subsequent changes in the office layout. Surveys show that more than 40 percent of the occupants in modern office buildings relocate at least once each year. Annual occupant relocation, quantified as “churn” rate, is increasingly common; it is also expensive, especially for high-tech businesses. In many cases, reducing churn-related expenses, such as rewiring costs, can repay the additional investment of installing a nontraditional floor.
If implemented properly, UAD buildings are designed and built for virtually the same cost as conventional designs, according to Tate. And there are added advantages: UAD systems provide an alternative method of delivering space conditioning and wiring in offices as well as other commercial buildings. Also, in addition to cost savings, UAD offers versatility in building renovation because all of the systems are easily available.
The fact is, when an experienced team incorporates UAD into a building design, the floor-to-floor height is reduced by 6 inches to 12 inches, noted Alwine. In a building with conventional overhead cooling, the floor-to-floor heights are usually set by the beam, duct interface at the core wall. Supply and return ductwork, and sprinkler pipes compete for space at the core, and generally dictate the amount of space required above the ceiling.
A typical ceiling plenum ranges from 20 inches to 24 inches below the structural beams. With UAD, there is no supply or return ductwork, which allows the ceiling plenum to be compressed to 0 to 8 inches below the structural beams. With UAD, the return can actually be in the sidewall of the core, eliminating the need for a return plenum.
“Over 50 percent of Tate commercial office projects are now using underfloor air distribution systems,” noted Alwine.
The complexity and the amount of data cabling have an impact on the height of the raised floor. Most buildings with UAD and a single level of underfloor data cable management have 14-inch raised floors.
PASSES THE TESTSThe key with HVAC systems, whether it is a standard overhead variable air volume system (VAV) or UAD (see sidebar), is how HVAC systems fare with regard to the typical complaints such as “too cold,” “too quiet,” and energy costs. Then there are other possible issues such as mold and other contaminants - will there be a higher risk?
Alwine was eager to tackle both issues.
He said conventional overhead VAV uses nominal 55°F supply air. This is a magic number in the design of cooling systems. At 55° supply air temperature, space conditions in the room are maintained at 72° and 50 percent humidity. Air must be cooled to 55° to wring out the moisture and keep the humidity levels acceptable.
UAD systems use nominal 63° supply air. If air is supplied from the cooling coil in the air-handling unit at 63°, there will not be enough moisture taken out of the air to keep the humidity within acceptable levels. The answer is to use a system called coil “face and bypass.”
Most of the air is passed through the cooling coil and cooled to 55° to wring out the moisture, and some of the air is “bypassed” around the cooling coil to blend with the 55° air to raise the temperature to the required 63°. Also, the outside air that is brought into the building to ventilate and provide fresh air has to be pre-treated to remove humidity. This approach makes the face and bypass approach more effective.
The reason for the higher supply air temperature with UAD is occupant comfort. Years of research and use in Europe have shown that 63° is the magic number. A big mistake made by inexperienced designers is not understanding the importance of the 63° supply temperature. Using 55° supply air similar to overhead VAV is a prescription for disaster. The second big mistake is using reheat to raise the supply temperature to 63°. It is not code-compliant and will result in huge energy bills.
With UAD, the air supply system delivers air at low pressure and low velocity. As such, there is none of the usual background noise associated with overhead VAV. If speech privacy is important, then an electronic white noise generator should be included in the tenant design. These systems are relatively inexpensive, and the noise generator and speakers are placed above the ceiling.
To date, Bick Group has not seen mold as a problem. Of course, the underfloor plenum must be extensively cleaned after construction. Mold requires moisture or elevated humidity levels. If the cooling system is designed properly to dehumidify, this is not an issue. The other potential source of moisture is a leaking pipe under the floor. To address this, leak detection systems must be used in specific areas.
Meanwhile, the main concern with life-safety issues and UAD is smoke, which might develop under the floor and be distributed into the space. Tate Access Floors is unaware of any instance where this has happened. A modern, code-compliant office building will have space smoke detectors, as well as a smoke detector in the mechanical rooms. There is some talk in the industry about adding smoke detectors under the floor, but so far this has not made it into model codes.
There is no question that the industry will continue to seek new and innovative methods that provide not only cost-savings alternatives but ease of use as well. For certain types of buildings, those that house office space for example, UAD offers a solid option that will serve not only the building owners but the tenants as well.
For a third-party opinion regarding UAD, see the related online feature article, “Pointing Out the Potential Advantages in UAD,” in this issue.
Sidebar: The Third Choice?Traditional overhead variable air volume (VAV) distribution is used extensively in office buildings. Supply ducts, VAV boxes, and overhead diffusers - usually in an above-ceiling plenum formed by a suspended ceiling - distribute cold, 50-55°F supply air to the spaces. This method of air distribution produces relatively uniform temperatures throughout the space because it induces significant mixing of space air with supply air. Return air leaves the space at approximately room temperature.
Displacement ventilation, on the other hand, is commonly used in industrial spaces, theaters, and other applications with very high ceilings. Diffusers, usually mounted low in sidewalls, release slow-moving, 65-72° air into the space; meanwhile, heat sources in the space induce local airflow from the floor toward the ceiling. Along the way the air stratifies into temperature layers, which become progressively warmer from the floor to the ceiling. Depending on the heat sources, airflow rate, and ceiling height, the air is 85° or more when it enters the return openings near the ceiling.
Underfloor air distribution (UAD) systems represent a third choice, “partial” displacement ventilation. Floor-mounted diffusers release cool 63-68° air, which induces local circulation and causes partial mixing and relatively uniform temperatures from the floor to a height of 3-6 feet (1-2 meters). Above the point, the air temperatures stratify. At the return openings near the ceiling, the air temperature ranges from 80-85°, depending on heat sources, airflow, and ceiling height.
Publication date: 06/09/2008