All new technology must go through its growing pains, and the use of underfloor plenums for HVAC distribution and power, voice and data cabling is no exception. While raised-floor systems have been used for space conditioning in commercial buildings overseas for more than 30 years—and for process cooling in computer rooms for more than 50—it is only recently that they have become more common in U.S. office buildings. Concerns still exist among code officials, building owners and tenants regarding the safety of these combined systems, and additional life safety devices such as smoke detectors and sprinklers can greatly increase the first cost of raised-floor systems. However, by increasing the level of communication with all parties involved, design engineers can head off many potential problems they might encounter.

A consensus solution is critical when designing underfloor systems, says Raj Gupta, P.E., president of Environmental Systems Design in Chicago. “Code officials are concerned about safety, but we’ve had great success when we’ve engaged them early, listened to their concerns and come back with solutions,” he says. “The last thing you want to do is proceed with a building and have problems during the occupancy phase.”

ESD has done a pair of office buildings employing UFAD in Chicago—Dearborn Center and UBS Tower. The firm is also installing the system in two other office projects under development.

Dan Murphy, P.E., Gupta’s colleague and a senior vice president specializing in codes at the firm, adds that his code experience has definitely been a learning experience.

“To me, it’s the logical movement of the future,” says Murphy of the technology, “but you have to sit down and slowly work it out instead of just throwing it at them.”

And be prepared to wait, advises the engineer, as he says it took eight months for the city of Chicago to grant the firm approval to proceed with UFAD its first time out of the gate.

The lack of familiarity with UFAD systems by all parties is a major challenge, but one that can be overcome, adds Fred Bauman, P.E., a research specialist with the Center for the Built Environment at the University of California at Berkeley.

“People are improving their understanding all the time, but it’s very jurisdictional,” Bauman says. “The first job in an area often sets the ground rules for what goes on, because it’s the first time the fire marshal has seen it. But, things are improving now, because we can point to examples of what others have done.”

Part of the confusion about fire protection for UFAD systems stems from past experience with raised-floor systems in data and telecom settings.

“When you say ‘underfloor system,’ most people only think about the computer room they designed 15 years before,” says Chris Schaffner, P.E., an associate with Arup in Cambridge, Mass. “Smoke detectors and halon systems, including heads under the floor, were used in those cases, but it was for protecting the equipment—not for life safety. They didn’t want to risk destroying the mainframe computer that cost millions of dollars to create.”

It really does boil down to overcoming the learning curve, adds Jeff Schumacher, a senior consultant in the Minneapolis office of RJA. “Many in the design community are encountering UFAD outside data and telecom for the first time,” he says. “But, UFAD is not necessarily any different than overhead air distribution. Turn the plan upon its head, and the rules are pretty much the same.”

What the codes say

To be specific, the main issue regarding fire protection in underfloor systems is flammability of materials. The National Fire Protection Association (NFPA), for example, addresses this in Chapter 4.3.10.6 of NFPA 90a: Installation of Air Conditioning and Ventilating Systems, 2002 Edition. Basically, any wire and cable in the plenum must be noncombustible or limited combustible and have a maximum smoke-developed index of 50, or tested in accordance with NFPA 262: Standard Method of Test for Flame Travel and Smoke of Wires and Cables for Use in Air-Handling Spaces, 1999 Edition.

The Uniform Mechanical Code, issued by the International Association of Plumbing and Mechanical Officials in Ontario, Calif., has similar flammability requirements. However, regulated use of UFAD systems is limited to residential applications. Use in commercial buildings, according to the code, requires an equivalency from the local authority having jurisdiction (AHJ).

Perhaps the biggest news regarding codes and UFAD is the recognition of plastic floor diffusers. Until the release of the 2002 edition of NFPA 90a, plastic diffusers were not approved for use with UFAD systems. This, according to Gus Faris, director of engineering at Nailor Industries Inc., Houston, was because of plastic’s inability to meet the 25/50 flame-spread/smoke-spread index testing requirements of NFPA 255 or UL 723.

“No plastic can pass this unless extreme amounts of graphite are injected into the plastic,” he says. “Injecting the required amount of graphite would cause the plastic to become brittle, and then it would not be suitable for use in a floor diffuser.”

Some code officials overlook the requirement and allow plastic to be used. Others are stricter and require metal diffusers, which are now available. The revised edition of NFPA 90a, however, subjects plastic diffusers to a new test for smoke generation—NFPA 262 or UL 2043—that involves burning just one diffuser vs. a larger amount of plastic. Some diffuser manufacturers have since complied by producing NFPA 90a-approved plastic diffusers, and more are expected.

“Because there is very little plastic in one diffuser, it is easier to comply to this standard,” Faris says. “The old standard was really for building materials and did not allow for the fact that, many times, the materials being examined might exist in rather small quantities as opposed to large amounts such as walls or floors.”

Beyond the flammability issue, rules are less specific. No mandatory requirement exists for use of sprinklers with underfloor systems. However, according to Gregory Harrington, senior fire protection engineer with NFPA, smoke detectors may be required if airflow exceeds 2,000 cfm. Detectors must also be properly matched to handle the level of airflow.

Also new to the 2002 edition of NFPA 90a (Chapter 4.3.10.1.2) is a requirement for removing abandoned wire and cable. The problem, according to Daniel Kaiser, engineering group leader with Underwriters Laboratory, Fire Protection Division, is that the new rule is difficult to enforce.

“You don’t need a permit to upgrade cabling in an existing building,” Kaiser says, “and, the only time an authority having jurisdiction can rule is if a permit is required. But, installers and specifiers should be pushing to have old wires and LANs removed.”

How much wire and cabling is too much is a question that researchers may address in the future, Kaiser says. “When these requirements originally were written, it was before computer networks and massive phone networks were developed,” he says. “The original codes didn’t have this mass of wire and cable in mind. The question is: Are we heading down a path where we’re going to have problems?”

According to Murphy, another trepidation on the part of local code officials stems from housekeeping fears. “Around eight years ago, we wrote to the city about the idea of putting in low-voltage wiring in ceiling return plenums, and we were approved,” he says. “Down the road, a building inspector happened to see an open ceiling in one of these buildings and found literally thousands of cables up there. That ended that experiment.”

Housekeeping, Murphy adds, is still not regulated by most local codes. In Chicago, a yearly inspection is required for UFAD approval, he says, and the underfloor plenum itself must also be broken into four quadrants equipped with smoke detectors.

Next steps

Besides Chicago, Milwaukee and Boston have joined the list of not-so-moderate-climate cities putting the technology in place.

“A few years ago, there was only one building in the Chicago area with an underfloor air-distribution system,” Gupta says. “Now there are five or six. In a few years, there will be even more.”

But until UFAD systems become more common, engineers will likely continue to face challenges and questions about UFAD systems, especially regarding fire protection measures.

In the meanwhile, cooperation and communication are the keys to success.

“Get the facts, analyze the risks,” Schumacher says. “And don’t ever assume that everyone is on the same page.”

Wireless the Way to Go?

Besides fire protection, another twist in the continuing saga of underfloor air systems is greater environmental control.

Of course, one of the big advantages of underfloor systems is that they can offer individual environmental control at workstations. Another major advantage is that cabling can be quickly and easily re-routed to new areas to accommodate churn. In putting together a building automation plan, churn can cause a few kinks.

In developing an office renovation of an old warehouse in Portland, Ore., Gerding/Edlen Development found wireless was the way to go. According to Dennis Wilde, a senior project manager with Gerding, instead of fixed thermostats, the firm placed a number of wireless zone sensor thermostats in various locations for local zone comfort control.

Like cabling, Wilde says the advantage of the scheme is that the wireless sensors can be relocated as the building floor plan is revised. The wireless approach was also consistent with the client’s—a prominent advertising agency—desire for an open floor plan without overhead cabling or conduit.

Apples to Apples, Plenum to Plenum

When it comes to energy-efficient technologies, most building owners say the same thing: Show me the numbers. But when underfloor air-distribution (UFAD) systems come up for consideration these days, engineers may have a new arrow in their quiver.

Researchers from the Center for the Built Environment (CBE) at the University of California Berkeley and UC San Diego, the Simulation Research Group at Lawrence Berkeley National Laboratory and York International are developing UFAD system simulation software modeling energy performance that allows an accurate comparison with conventional HVAC systems. Ultimately, the UFAD modeling capability will become a part of EnergyPlus, the whole-building energy simulation program developed by the U.S. Dept. of Energy (DOE) based on DOE-2. Work is underway at three test facilities to address the combined effects of underfloor plenum performance, thermal transfer between supply air and slab and room air stratification.

But as work on the tool will continue for two more years, engineers may seek a more imminent resource: the ASHRAE Design Guide on Underfloor Air Distribution Systems, which is expected before the end of the year. The guide offers a standardized plan for UFAD system design and task/ambient conditioning based on research conducted by UC Berkeley’s CBE.

“We’re presenting the latest information we have,” says CBE’s Fred Bauman, P.E., primary author of the guide. “But, we need to know more, and our research will continue.”

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