HVAC: Underfloor Air Systems
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Underfloor air systems, in which raised-access floors serve as plenums for distributing cooled air throughout buildings, are growing in popularity in North America. The potential benefits of underfloor air distribution include improved thermal comfort, improved indoor air quality, reduced energy use, and improved flexibility for office moves. Although slow to catch on in North America at first, the burgeoning underfloor air distribution industry is feeding itself in an upward spiral today. As more systems are installed, manufacturers have more examples to point to when trying to sell builders and designers on the technology. The knowledge base is also growing, giving engineers and designers more information and confidence that they can produce successful projects. At the same time, new products are being introduced and new manufacturers are entering the market to take advantage of the growing demand. On the user side, many companies today are looking for ways to cut the costs associated with frequent office reconfigurations. And underfloor air systems, together with raised floors, are helping them do just that.
What Are the Options?
There are two primary ways to distribute air with raised floors:
Pressurized floors operate with a small positive static pressure in the floor plenum—typically between 0.03 inches water gauge (wg) and 0.10 inch wg. This pressure drives the supply air through simple diffusers placed as necessary in the floor—typically one diffuser for each 100 square feet of floor space. These diffusers, which deliver air in a swirling pattern that is intended to mix supply air with room air, are frequently user-adjustable. This enables a building occupant to have a high degree of control over the temperature and airflow in the workspace.
Even with so little static pressure, air can be moved to diffusers at least 30 feet from a supply riser or duct without creating temperature inconsistencies in the space. Even longer distances are possible with deep floor plenums.
Zero-pressure floors rely on small fan-powered distribution boxes to push air up into the conditioned space. Some designs with fan-powered boxes in the floor keep the plenum at negative pressure relative to the space, to draw return air back into the supply air and moderate its temperature. Such systems are usually thermostatically controlled, as opposed to the manually controlled swirl diffusers that are more common.
If necessary, these two system types can be combined. For example, conference rooms or perimeter spaces far from the core supply risers can use fan-powered boxes and open interior zones can use simple floor outlets.
Figure 1 shows typical floor-to-floor sections for conventional and underfloor air-conditioning systems. A typical raised floor uses square cells on a 2-foot-by-2-foot grid, with support columns at grid intersections. If greater structural stiffness is required, stringer beams can be added to the floor system, but floors without this extra support are surprisingly stable. Building codes in some cities may require that the floor tiles be mechanically anchored to the support columns with screws-an addition that adds about $1 per square foot to the installed cost.
A raised-floor system can be installed in the same vertical space as a ceiling-based system. Some designs may even increase the clear space, as shown here.
Source: E Source
The critical dimension is the vertical distance between the subfloor and the raised floor, which varies from as little as 4 inches in some Japanese designs to more than 2 feet for systems that require underfloor ducts, fan-powered distribution boxes, or long throw distances. Most systems without ducts are between 12 and 18 inches high.
In addition to largely ductless passive designs, two other variations of underfloor systems are worth noting:
Displacement ventilation systems move large quantities of air through a perforated floor. This produces a laminar, vertical airflow from floor to ceiling, generally resulting in stratified air temperatures. (Displacement systems can also be installed without a raised floor, using large low-wall grilles or pedestals to let the air flow out onto the floor.) Heat sources in the conditioned space (such as people, computers, or copy machines) convect supply air upward in well-defined plumes that provide cooling where it is needed. Displacement systems have logical applications in open areas, industrial spaces (such as clean rooms), or for spaces with high pollutant loads (such as smoking lounges).
Task-ambient conditioning systems use an underfloor supply plenum to drive supply air directly to the occupant through floor-based, desk-based, or furniture-based diffusers, creating user-adjustable “task” conditioning. Widely spaced floor diffusers provide “ambient” conditioning, often at more economical temperature setpoints. In some systems, underfloor supply air is used for task conditioning while a conventional duct system in the ceiling provides ambient conditioning. The potential benefits of even slight improvements in worker satisfaction can dwarf energy savings—or even entire energy budgets.
As an alternative to underfloor air, so-called "thermal displacement ventilation" (TDV) systems are increasingly popular in school applications where a raised floor is not desirable. Such systems deliver air at floor level from wall-mounted horizontal diffusers. Air is delivered at a low velocity—typically 50 to 100 feet per minute—resulting in a quiet, low pressure system. Air is exhausted from the room at the ceiling. The overall airflow pattern of supplying low and returning high promotes thermal stratification (allowing hot air to rise). As a result, most of the heat gain from people, lights, and computers is drawn out of the space and exhausted from the classroom, greatly reducing the cooling load and airflow requirements. Because of the high minimum ventilation rates for classrooms and the reduced total air delivery requirement for TDV systems, it is common for such systems to use 100 percent outside air, which provides the significant advantage of not recirculating room air. This greatly reduces airborne dispersion of germs.
How to Make the Best Choice
Evaluate cost-effectiveness. Underfloor air systems may or may not have greater initial costs than more conventional systems—field experience varies on that score. But a well-designed underfloor system should be less expensive to own and operate over its lifetime. For example, for one user, costs associated with moving employees around decreased from $450 per move per individual to $100 after underfloor air and raised-floor systems were installed. Reconfigurations now consist only of furniture moves and no longer require electricians, telecommunications and IT specialists, and mechanical contractors.
Energy savings from the use of underfloor air systems result from reduced fan power, higher chiller efficiency, and the ability to use economizers over a broader temperature range. Increases in productivity and decreases in absenteeism can lead to significant savings but are also hard to quantify. Recent studies at the Center for the Built Environment (CBE), a National Science Foundation/Industry/University cooperative research center at the University of California at Berkeley, indicate that giving individuals control over local heating and cooling, by underfloor air or other means, can improve productivity by 3 to 7 percent.
Consult with the right sources. One of the limiting factors for underfloor air systems in the mid-1990s was a lack of knowledge about and experience with such systems. There is a natural tendency to go with the tried and true so that engineers can be confident a system will perform as expected and the engineering firm will not be hampered by additional legal liability. In recent years, as more underfloor air systems have been installed, engineering firms have gained more confidence in the technology, and more information on how to properly size and install these systems has been disseminated.
Although manufacturers themselves have developed their own guides and procedures, a lot of the new information available has come from CBE. The center has published several research reports and design guides and maintains a Web site about underfloor air technologies.
Make sure contractors are familiar with the technology. The construction community has been on a learning curve with underfloor air technology. The construction process for underfloor air requires that steps be taken in a different order than for conventional HVAC systems. For example, in the underfloor air process, the building’s interior can’t be worked on until floors are completed, and the building has to be enclosed earlier than it would in the conventional process to protect the floors.
Get local code officials on board. Local building codes don’t mention underfloor air systems specifically, but these codes can sometimes be interpreted in such a way as to rule out underfloor air or to make it more expensive than necessary (for example, by requiring that the underfloor plenum be provided with fire sprinklers in addition to the overhead sprinkler system). Because of code variations, some designers always talk with local officials at the outset of a project and put it on record that the floor has been approved. Otherwise, the builder is at the mercy of the field inspector.
In multistory buildings, it is important to pay attention to insulation between floors. The heat from light fixtures as well as generally hotter temperatures near the ceiling will cause heat transfer to the floor above. If proper insulation is not provided, this can lead to unacceptable temperature increase in the underfloor plenum that is cooling the building level above.
What’s on the Horizon?
Several new products have emerged in the past few years, giving designers more choices than ever before. These have included slimmer, quieter fan boxes and improved, easier-to-use diffusers. New fan terminals have also been introduced that fit beneath a single-access floor tile, eliminating the need to use large units originally intended for ceiling-based HVAC systems. Look for more products with greater controllability and slimmer profiles in the future.
Copyright 2006 - Platts, a Division of The McGraw-Hill Companies, Inc.