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 in the floor—typically one diffuser per 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 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 a deep floor plenum.
Zero-pressure floors,, on the other hand, 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 more common manually controlled swirl diffusers.
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 hows 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.
Figure 1: Vertical dimensions of conventional and underfloor air-conditioning systems
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.
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 and task-ambient conditioning systems.
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.