Underfloor air distribution 101.

For years, European engineers have used floor plenums created by access-floor systems to provide building conditioning. It wasn't until the mid-1990s that underfloor air distribution (UFAD) systems gained in popularity in the United States. Their use is becoming

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Raised Access Floors

A raised access-floor system consists of a rigid structural grid that supports lay-in floor panels. These floor panels are typically 1- to 1.5-inches thick and constructed of reinforced steel or concrete-like material. They're supported so that the resultant floor level is typically 12-inches to 36-inches above the slab, creating a cavity in which power supply, data, and communication cables can be located. These cables are routed via modular connections to floor or system furniture panel outlet boxes. Space changes can be easily accomplished by relocating floor panels and adding or deleting modular sections of cable.

The resulting underfloor pathway created by the raised floor panels provides housing for any type of service distribution system, modular wiring, passive or active zone cabling, and HVAC service. To achieve this, the modular panels that make up the access floor can be equipped with power-voice-data (PVD) termination boxes to provide convenient, flexible access to these services. Plus, the PVD boxes can be relocated anywhere desired as spatial layout or telecommunications needs change. Modular and relocatable variable air volume (VAVI diffuser terminals or passive air diffusers provide increased occupant comfort and individual control of air volume.

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UFAD Basics

There are two primary ways to distribute air within raised access floors: pressurized floors and zero-pressure floors.

Pressurized floors operate with small amounts of air pressure in the floor plenum--typically 0.10-inch water gauge (wg). This pressure causes conditioned air to be pushed through floor registers or diffusers to the space or room above. Typically, one diffuser for every 100 square feet of floor space is used. These diffusers deliver air in a swirling pattern that's intended to mix with existing room air and provide heating or cooling.

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 swift diffusers that are more common. If necessary, zero-pressure floors and pressurized floors can be combined. For example, conference rooms or perimeter spaces that are far from the core supply risers can use fan-powered boxes; open interior zones can use simple floor outlets.