What Is Underfloor Air Distribution?

Established in May 1997, at the University of California, Berkeley, the Center for the Built Environment (CBE) provides timely, unbiased information on promising new building technologies and design techniques, including underfloor air distribution systems.

When it comes to underfloor air distribution, David Lehrer, R.A., CBE's director of partner relations and communications, is in the know. Here, he shares the organizations knowledge and experience with the system, starting with a definition: "Underfloor Air Distribution, known as UFAD, is a system of supplying conditioned air from a pressurized plenum that is created between the floor slab and a raised floor system."

System Differences

There are a number of ways in which UFAD is different from a conventional overhead air system. The first is that air moves from a bottom to top manner because it is supplied at low levels directly into the occupied portion of the space. This also means that conditioned air and outside/fresh air is provided much closer to occupants. Another difference is that air is supplied at low level and removed at high level. A third difference is that air can be supplied at a warmer temperature (63 degrees F to 65 degrees F) than it can with an overhead system (55 degrees F to 57 degrees F). Additionally, it's a low-pressure system: "The air is not forced through branching supply ducts," says Lehrer.

System Benefits

UFAD has many benefits, adding up to a worthwhile investment.

Plug-and-play: It is a plug-and-play system, which eliminates wiring in the walls. This makes it especially useful in facilities where you may change the layout of an open floor plan as teams reorganize and shift size. In this case, inhouse facility staff can often shift system components overnight.

Energy savings: With an underfloor system, air is provided at a higher temperature and, because the air doesn't have to be cooled so much, energy is saved from better chiller efficiency. In some climates, an economizer cycle can be used for many hours during the year, which is using outdoor air as opposed to the chiller to cool the building.

Energy savings are also realized in that the air supply requires less pressure, reducing the fan energy needed. Note that, even with lower supply pressure, comfort in the stratified zone is still maintained.

There's yet another way to save energy with UFAD. "The system supplies air directly in contact with the floor slab, so you have all that thermal mass available," says Lehrer. "It's a little tricky to do the controlling strategy correctly, but you can use a night-time flush process and bring in cool air at night — when electricity is cheaper — to run the fans and precool the slab."

To capitalize on energy efficiency, CBE is working on a three-year project with Lawrence Berkeley National Labs and the University of California, San Diego. The trio is developing an underfloor module for EnergyPlus, a software program used by energy professionals to model the energy performance of an entire building in the design process. "In the future, designers will be able to model UFAD systems on the computer and design them to be more energy efficient," says Lehrer. "That, I think, will be of great value to the building industry."

Indoor air quality (IAQ): With this benefit, the principle is that IAQ is improved by providing air closer to the occupants and letting the heated air and pollutants rise to the top, where they're removed from the system.

Building occupants typically respond favorably to UFAD, as CBE has learned through building occupant surveys. "With an underfloor system, the perception is that the air quality is better," Lehrer says. "We have determined that seeing a floor diffuser and feeling the air come up near them gives occupants a perception of better air quality."

"CBE would like to do more research to really measure the amount of improved air quality in the space," says Lehrer. "But everything points in that direction."

Sustainability: Administrators looking to gain LEED points in their next construction project will be happy to know that they can earn them with UFAD. "Energy efficiency can provide up to 10 points, so a carefully optimized system may be able to get many points in this category," Lehrer says. "Other points specifically attributable to (correctly designed) UFAD systems include one point for increasing ventilation effectiveness and one point for controllability of systems."

System Cost

There is some controversy about how much more, if any, UFAD systems cost than conventional overhead air systems.

The system includes, first of all, the raised floor itself, which has an installed cost of about $6 per sq. ft. Mounted in the floor panels are the air diffusers, of which there are three different kinds from which to choose: Variable Air Volume (VAV), passive swirl and linear (the latter is only for perimeter zones). Lastly, the system includes power/voice/data boxes that are connected with a system of modular wiring cables.

Lehrer notes that the system does tend to have slightly higher first costs. However, he also points out that integrated design reduces the additional cost to a fairly small amount. "Some design teams have been able to integrate the system with the building's structural system because UFAD requires going about things in a different way," he says. For example, in some cases, the floor-to-floor height can be lowered because there are fewer conflicts between supply ducts, structural beams and other services. Also, you may be able to downsize the chiller and fans, both of which can offset first costs.

When it comes to comparing first cost to lifecycle cost, initial research shows that underfloor systems provide savings from improved flexibility, thus reducing costs associated with office reconfiguration, or churn. Lehrer points out that CBE is beginning a detailed study of lifecycle costs, so more information should be available in the near future.

Helpful Hints

Whether you're convinced that UFAD is right for your project, or you still want to learn more, Lehrer offers some tips.

First, know that UFAD works best in new construction because, when implementing it in a retrofit situation, you have to be concerned about how raising the floor height affects where elevators and stairs land.

Also, the system works for heating and cooling, but CBE has done most of its work with cooling "because most large commercial buildings are in a cooling mode much more than they are in a heating mode," says Lehrer. He believes that heating with UFAD is acceptable because people tend to be more comfortable when they have warm feet and cool air around their heads. He points out that CBE doesn't know if UFAD offers the same energy-saving benefits with heat that are realized with cooling.

As with any building system, integrate it early in the building process. Lehrer encourages administrators to hire a mechanical engineer and possibly also an architect who have experience with these systems. "When asking for proposals, ask the design team to show relevant design experience with underfloor air projects," he stresses.

If you do go with UFAD, it's important to get buy-in from all project participants. "It's an educational process to get all the players on board," Lehrer says. "If someone really doesn't want to do it, he'll find a reason to support not doing it, so education in knowing how to take advantage of the benefits is important."

Finally, he recommends looking at existing case studies. Some schools that are already using UFAD include West Orange Charter Elementary School in Oakland, Fla.; and Detroit Public Schools (six buildings) in Michigan. In addition, some commercial projects are highlighted on CBE's Website: .

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