More commonly associated with data centers, underfloor air systems have started to penetrate the office market. This begs the question whether this is a legitimate trend or a case of a client or designer falling in love with a particular technology.Alisdair McGregor, a principal in the San Francisco office of Arup, one of the leading mechanical engineering firms in the application of unde...
More commonly associated with data centers, underfloor air systems have started to penetrate the office market. This begs the question whether this is a legitimate trend or a case of a client or designer falling in love with a particular technology.
Alisdair McGregor, a principal in the San Francisco office of Arup, one of the leading mechanical engineering firms in the application of underfloor air systems, agrees it's a valid question. "Arup has definitely seen an increase; however, that could be due to the fact that clients are seeking us out more because of our expertise," he says.
Among the many office projects with underfloor air the firm has designed—including facilities for General Motors and the state of California—is 901 Cherry St., the San Bruno, Calif. headquarters of clothing retailer The Gap. Granted, the facility was a "green" demonstration project (April 1999, p. 38), but its continuing successful operation has lead the clothier to make underfloor air distribution (UFAD) the standard for all of its office buildings.
Mark Husser, architectural director for St. Louis-based HOK, agrees that the technology is not just for data centers anymore. "Cabling and power have always been the traditional drivers, but clients are starting to find other factors," he explains. "Life-cycle cost, reduced equipment costs and chiller capacity are coming into play."
As an example, Husser points to the General Services Administration (GSA)—the federal agency responsible for building and managing courthouses and office facilities. HOK is currently designing the new Denver federal courthouse with UFAD. "GSA, for some time now, has recognized the benefit of access-floor systems in offices, not just because of cabling concerns, but also churn rates. Over time they've found this to be much more efficient," says Husser.
"Flexibility is really the key," adds McGregor. "Tenants want to be able to reorganize a space without having to rip apart a ceiling. Obviously, it's not such a major advantage for occupants who aren't going to change much or have a small floor plate to begin with."
It's also a much cleaner technology, according to Husser. "Traditionally, you have particulate being displaced around a space when air is introduced at high volume. But the warmer air from the underfloor plenum tends to carry particulate off where it can be filtered before being reintroduced," he says.
Brian Benson, P.E., a senior mechanical engineer with Minneapolis-based A/E Ellerbe Becket, has designed a number of data centers, and recognizes the advantages of underfloor air, but is not so sure about the upside of its use in the office market. "We've talked about it for office applications for years. Beside the cost and unfamiliarity on both the owner and contractor side, there's a lot of paranoia about leakage," he says.
In other words, Benson says, owners are afraid the underfloor might create pockets of dust or their employees might spill things like coffee and create a nasty mess. "It's really more of a housekeeping issue, but it's there."
Furthermore, he says that many corporations simply tend to be conservative with no desire to experiment. Additionally, they've encountered resistance from other members of the design team over seemingly trivial things like carpeting. "Interior designers are not the most practical people, and they take great offense when they are asked to use 24- x 24-in. carpet tile," says Benson.
While he acknowledges that such mind-sets exist, McGregor also knows that there are many corporate clients who are eager about using UFAD for reasons other than cabling.
"For a lot of clients, an important goal is creating an environment where individuals have control over their own environment," says McGregor. "It's really perceived as a big self-satisfaction item. That was particularly important to The Gap."
Such benefits should not be discounted, says Fred Bauman, P.E., a research specialist with the Center for the Built Environment (CBE) at the University of California, Berkeley. "Our initial research shows these kind of things are very valuable to employees," says Bauman. "The analogy of having an operable window is a good example."
Individual control, without question, is very much psychological, according to the researcher, and comfort is truly a state of mind, but it can be a factor in establishing higher productivity. "Comfort is not just 75°F and 50% relative humidity. It's how the individual likes it," says Bauman.
But the real challenge is quantifying these productivity and comfort gains. McGregor admits that Arup's UFAD clients are usually owner/operators who give more credence to such "fuzzy" benefits.
The question is whether more owners and developers will seek out such benefits. Some signs indicate that this is occurring. Husser points out that several developers are employing UFAD as a competitive marketing advantage—Beitler in Chicago, for example, is marketing its new Dearborn Center as "one of the most technologically advanced buildings being constructed in the world." Houston-based mega-developer Hines has also used UFAD on a number of projects, most recently for its Woodfield Preserves office complex in suburban Chicago (Oct. 2001, p. 64).
"Many clients see it as a big advantage for recruiting and retaining employees," says McGregor. "Hines was one of the first big developers to embrace the idea of a raised floor for spec buildings, and I think they have to because it's becoming something that many tenants are asking for."
No simple solution
That being said, UFAD systems are still something that should not be taken lightly. "Raised floor in data centers and computer room are a completely different animal than they are for general office areas, and there's a lot of misconceptions about that," warns McGregor.
Tim Lang, a mechanical project coordinator with HDR, Omaha, Neb., agrees. HDR employed UFAD for the First National Bank of Omaha's data/check-processing center that also included offices. "For general data centers and computer equipment rooms, you typically have 2-ft. x 2-ft. floor-panel diffusers, the supply air comes in at 55°F and there's no need for mixing to raise the incoming air to a more comfortable temperature," he explains.
"That doesn't work with underfloor systems in an office environment," adds Benson, "because if you deliver air that's too cool and too close to people, they're going to complain."
As a result, a UFAD system designed by someone more familiar with top-down airflow design could actually end up compromising occupant comfort. Bauman likens the situation to the introduction of variable-air-volume (VAV) technology a couple decades back, where many designers didn't fully understand how to design and operate them.
Of course, a big part of successful UFAD implementation is the concept of stratification, where supply air, commonly introduced at about 63°F, rises slowly and mixes with the heat generated by people and office equipment, such as PCs, until a comfortable temperature is achieved, according to Bauman.
The problem, according to Bauman, is that many people don't understand stratification. "Obviously, if you keep blowing in air, you'll get a well-mixed space. But if you reduce the velocity and volume of air, you can achieve some real energy savings in the form of less fan energy by letting the natural buoyancy of the air keep the areas at the lower level comfortable, purposefully letting the upper level get hot and nasty."
Low volume distribution, of course, is a variant of displacement ventilation. But with true displacement ventilation, according to Bauman, air is literally "oozing" out of diffusers. And to achieve the 63°F entry air which is common in most office UFAD applications, some mixing is still required.
"And for an office environment, I would say you probably need more perforated floors like you see in file-server rooms vs. the individual diffusers you see in most office UFAD applications," says Lang.
Still, Bauman says low-volume UFAD schemes have their place, particularly in climates that don't see wild fluctuations in temperatures, such as Northern California. In fact, in such cases, there is great potential for energy savings, as more outside, unconditioned air can be employed year-round.
In fact, according to CBE data, as much as 60% can be saved with a low-volume VAV operating at 90% cfm at a 50% design load (relative to an overhead VAV system); even at the low end, at least 10% savings can still be accrued, even if operating a VAV at 120% cfm and 100% design load.
"You really don't want to put in too much air," McGregor concurs, adding that this was a particular problem in some of the early UFAD office projects. "People would get cold and dial down their dampers or close them off completely, and it really became a cascading effect."
Besides controlling the volume and velocity of air, other control measures can be put in place to prevent UFAD systems from malfunctioning. For example, making dampers impossible to close—at least in common areas—can prevent the system from being shut down or drastically altered as a whole.
Bauman, along with other researchers at CBE, have assembled the essential underfloor air technology website: www.cbe.berkeley.edu/underfloorair
On the site, multiple UFAD strategies and case studies are spelled out in detail. Briefly, typical UFAD strategies involve bringing air into the plenum and distributing it passively, via swirl diffusers, or actively, by fan-powered units that can be controlled by the occupants (see Figure 1, p. 29).
Regardless of the means, supplemental heating/cooling is usually required for perimeter areas of a UFAD space. This usually takes the form of radiant heating or fan-powered terminal units, often including reheat elements (see Figure 2, p. 30).
These perimeter areas, which are often low-density environments, are typically partitioned in the plenum from higher density areas. Ellerbe Becket is trying to go the opposite route for a UFAD office project in Virginia. "We're trying not to have any partitions below," says Benson.
The job—a demonstration project for trying to accommodate smokers and non-smokers in a work environment by using 100% outside air with no recirculation—is using an 18-in. raised floor with VAV terminals mounted in the plenum.
Additionally on this project, the firm is moving away from tenant-controlled diffusers, instead opting for thermostatically controllable VAV units, which will make the system more automatic. This, of course, removes some of those "fuzzy" benefits, but was a choice Benson felt they had to make in the greater scheme of the project.
"People still want zone control, but it can become problematic in an underfloor scheme because you don't want to end up with a wild system. To prevent that, you would need to install partitions or channeling, and that defeats the whole point of the underfloor system, which is to provide flexibility for future wiring changes."
Not so expensive
Regardless of the pros and cons of a particular technology, most often it's the bottom line that drives its adoption. The cost of a raised floor alone is typically about $6 to $8 per sq. ft.—still a prohibiting factor. The good news for UFAD proponents, is that its practitioners says system-wide costs, including life-cycle costs, can be brought within reasonable levels.
"When you eliminate chiller capacity, ductwork, some of the equipment, and sometimes even the footprint by using a raised floor, you can get the cost down to about $3 to $4 per sq. ft.," says McGregor.
And, of course, as Benson points out, more people in the industry actually design and install these systems, particularly large users like Hines or The Gap tend to go back to it. "They know how to build it, use it, have a price history and want to use it more," says Benson. "It's an educational curve, including the need to show clients what they can eliminate by going this route—like getting rid of ceiling spaces and related ductwork. It's the same with component sizing, such as AHUs or rooftop units."
One side benefit, at least on the construction side, says Benson, is that the concrete slab doesn't have to be perfect. This factor, however, can have a major impact on cost and schedule.
Ultimately, UFAD in offices still has a long way to go, especially because there is no major cost or energy savings compared to traditional overhead HVAC. But there is hope.
"The concept is rising or at least being thought of more," says HDR's Lang. "But I'm still not sure anyone is doing it just for the HVAC benefits alone."
"I definitely feel it's a superior technology," says Benson. "And it will become more popular in office settings because tenants will want the flexibility. Oftentimes, that's the driver. It's an emotional decision— 'I want the building to be to be green.'"
McGregor also believes the technology has a future, pointing out UFAD is the standard in the United Kingdom.
Its immediate use, however, is even more uncertain. "The problem right now is that there are not a lot of people building offices," he says.
Manufacturers Believe in UFAD
One gauge for measuring the potential for underfloor air in the office market is the manufacturing community. HVAC giant York is certainly a proponent of underfloor air, and has been touting its Flexsys system for some time now. York, in fact, is on the cutting edge of a number of underfloor applications. It is presently working with select clients to promote the technology as a solution to accommodating people in traditional smoking environments, such as in restaurants and hotels.
Schofield, Wis.-based fan manufacturer Greenheck is also a believer. After working closely with firms like San Francisco's Arup on a number of raised-floor office projects on the West Coast, the fan manufacturer went to the drawing board to produce what they feel is a fairly unique VAV/reheat terminal unit. Its UFT fan was custom designed to fit within the underfloor support pedestal scheme. Beside adaptability, the unit, just introduced to the market, offers up to 1,798 cfm—almost twice the air volume of most VAV units.
Looking at the national construction markets, the company has found the San Francisco/Northern California area to be hot. According to Dave Kallstrom, a sales and marketing manager for Greenheck, it may have to do with the Bay area's steady climate. "You can run an economizer more or run the unit at night to cool the concrete slab and retain that cooling," he explains.
As far as his personal take on the state of underfloors in the office market, Kallstrom says that it's a little slow, but good things are happening.
"Engineers are conservative by nature, and are hesitant to downsize fans and other equipment. But because of the temperatures in places like San Francisco, you can get underfloor costs down to about $3 a sq. ft., which makes it more competitive."
Furthermore, because of places like the Center for the Built Environment at the University of California, Berkeley, he says there will continue to be a lot of evaluations of these projects for public consumption.
"Hopefully this should lead more improvements in the technology," says Kallstrom.
Radiant Ceilings: Another Alternative HVAC Solution
Another alternative HVAC technology surfacing more on the market is radiant ceilings.
The technology can be used for either heating or cooling, and typically tends to be applicable more for specialty situations, medical in particular, according to Rick Hermans, P.E., a medical system specialist with Ellerbe Becket, Minneapolis.
"Their biggest use is in very cold climates where some kind of supplemental heat is needed to offset heat loss due to large windows," he says.
The big advantage of putting the panels in the ceiling vs. the floor, is cost. "They don't have to be set in concrete like traditional radiant heating, so if there is ever any problem with the tubing, you don't have to bust it out," says the engineer.
Additionally, these systems tend to be a little cheaper than conventional radiator systems, and in many facilities like hospitals, concerns about dust and mold have led to prohibitions against certain finned-tube piping.
But there's also an architectural component. "If some perimeter heating is necessary, but the architect doesn't like the look of a radiator, this is a great option because the panels have some very aesthetic qualities," he says.
On the negative side, Hermans admits these systems tend to be better for heating, as they don't offer the same efficiencies for cooling a space. However, the panels can be used independently.
But the biggest drawback is condensation. "Sometimes it can literally rain in a space if you don't design it carefully. This has actually been quite a detriment," says the engineer.
But, according to Hermans, in the right circumstances, with the right design, radiant ceilings can be a good solution.
"They're easy to clean, very washable and antiseptic. That's why they're nice for hospitals," he says.