Underfloor-air-distribution (UFAD) systems have been used for comfort conditioning in U.S. office buildings since the early 1990s. With a UFAD system, air is delivered to occupied spaces through floor-mounted outlets supplied by a pressurized plenum beneath a suspended floor.

A properly designed UFAD system takes advantage of thermal stratification. The key is to have a diffuser that rapidly mixes air before it penetrates the stratification layer at the upper end of the occupied zone. The pressurized plenum (the area between the slab and the raised floor) essentially is a large duct maintained at a constant pressure differential — typically, 0.05 to 0.10 in. w.g. — to the room above. This pressure is maintained through the supply of conditioned air from a number of supply-duct terminations. The spacing and location of the ducts is dependent on air-supply requirements and plenum depth (typically, 12 in. to 24 in.). If zone control is desired, the underfloor plenum can be partitioned into separate zones. Return air should be located at the ceiling or a high sidewall. This prevents heat from ceiling lights from entering the occupied zone.

Concerns

Concerns typically associated with UFAD systems include:

  • Humidity

    With the higher supply-air temperatures (most commonly, 60˚F to 65˚F) used in UFAD systems comes possibly higher moisture content. The supply system must reduce relative humidity to less than 60 percent to meet indoor-air-quality concerns, which requires dew points of less than 65˚F. Potential solutions are reheat or the blending of air to achieve a 65˚F-supply/55˚F-dew-point condition. System designs utilizing condenser-water reheat, runaround-coil face and bypass, and other strategies can be employed. Other options include the use of a separate system to dry outside air and the use of desiccant dehumidification.

  • Dirt and spillage

    Although UFAD systems are not recommended for areas with high potential for spills, such as bathrooms, cafeterias, and laboratories, small spills are not a problem for most applications. Typical swirl diffusers have a receptacle to catch spills and dirt from normal daily use. The receptacle, which can be removed and cleaned, has a basin that will hold anywhere from 4 to 6 fl oz of liquid.

  • Leakage

    Leakage typically is the result of poor sealing or construction quality at window/wall locations, stair landings, electrical outlets, etc. These areas have to be sealed and framed so supply air does not travel up walls toward return air. Leakage between floor panels can be reduced by staggering carpet tiles over floor tiles. The key is to limit the number of penetrations into a raised floor, which reduces the number of areas in need of sealing.

Benefits

Because floor-plenum pressure typically is less than 0.10 in. w.g., energy-efficient, low-pressure fans can be used. In place of complicated and expensive duct systems supplying air to individual outlets in a ceiling system, UFAD systems deliver air to building zones using a limited amount of ductwork.

Whereas traditional overhead mixed systems provide conditioned air from floor to ceiling, partially mixed systems such as UFAD save energy by providing comfort conditioning in lower occupied spatial zones and allowing upper zones to stratify.

Building Core

In the core of a building, where loads are relatively constant, round (swirl) or rectangular outlets are located in the floor near occupants. These outlets typically deliver 80 to 100 cfm of conditioned air to spaces. Swirl diffusers typically are available with a flow regulator adjustable manually by occupants or through the use of a room sensor connected to an actuator mounted on the diffusers. Installation of swirl units has been made easy through the replacement of mounting rings, which previously were attached to units beneath floor tiles, with spring clips, providing a press fit directly into floor tiles.

An ASHRAE research project (RP-1373) provided data showing that when the height of an air plume to a terminal velocity of 50 fpm is limited to 4.5 ft, breathing-zone air-change effectiveness is improved. For these conditions, Addenda a in Table 6-2 of ANSI/ASHRAE Standard 62.1-2010, Ventilation for Acceptable Indoor Air Quality, allows a zone-air-distribution-effectiveness rating of 1.2. This means the ventilation (outdoor) air supplied to a zone can be reduced by 16.7 percent. For Leadership in Energy and Environmental Design (LEED) projects, this 16.7-percent reduction can be used to reach the goal of 30-percent increased ventilation air required for Indoor Environmental Quality Credit 2.

Perimeter Loads

Some of the biggest challenges facing designers of UFAD systems concern perimeter loads, which are higher and fluctuate because of effects of radiation and temperature conduction on the skin of a building. Whereas the core of a building is impacted mainly by a nearly constant heat load, a perimeter system must accommodate what can be rapid swings in heat load and heat loss.

A common method of handling perimeter loads is to locate a fan-powered terminal in a floor plenum near a building's perimeter and duct it to outlets located on the perimeter. A typical outlet would be a linear bar grille with a boot plenum or a continuously fed plenum underneath. Equipped with an optional hot-water or electric heating coil, a fan unit can deliver warm air in response to a space thermostat. Unfortunately, as linear grilles get longer, discharge air is projected higher than required. If the throw from the outlet is too long and reaches the ceiling, it may deflect downward into the space, creating unwanted drafts. In some cases, cool air from floor plenums is supplied to perimeter zones through fan-powered units as well.

The operational cost of running a fan-powered terminal can be minimized through the use of electronically commutated fan motors. Electronically commutated motors (ECMs) operate at an efficiency of 70 percent or greater. The cooler operation and enhanced construction of an ECM contribute to longer life and lower life-cycle cost, compared with standard-construction permanent-split-capacitor motors. An additional benefit of an ECM is the ability to control fan speed to increase energy savings and improve occupant comfort. ECMs can utilize remotely controlled speed controllers, which can be controlled through a building-management system.

New Technology

New technology can lower installed and operational costs and improve comfort in perimeter zones. A continuous bar grille can be installed along a perimeter, with variable-air-volume (VAV) cooling and plenum heating coils attached as needed to condition the perimeter. These cooling and heating units are passive and do not require the use of a fan terminal. A bar grille can be installed to provide a continuous architectural appearance around a perimeter or installed in sections as required for comfort conditioning.

VAV cooling units employ an electrically actuated sliding damper that opens and closes a series of transverse apertures to vary the volume of cool air supplied from a pressurized underfloor plenum to a space. The damper opens and closes to provide the amount of conditioned air required to manage changing conditions, as directed by a room thermostat located in the occupied zone. The transverse apertures manage supply air to allow room air to be induced into the air pattern. Introducing supply air in small amounts helps to manage the projection from the outlet and prevents long throws, which create drafts in occupied space.

Located parallel at the glass on a building's perimeter, heating plenums mix cool convection currents flowing down the glass with warm air currents traveling across the floor. These mixed currents are induced into the inner chamber of the plenum and flow up through the heat exchanger. The warm currents then exit the linear grille at the glass and flow upward via convection to heat the cool air in front of the glass.

The heating units have a finned-tube heat exchanger, with heat supplied through a hot-water pipe and controlled by a water valve to provide the precise amount of heat required to satisfy room conditions.

The modular construction of VAV cooling and fin-tube water and electric heating units allows installation to match the requirements of any climate zone. Where winter conditions prevail, more heating units can be installed to meet heating needs. Where hot summer conditions prevail, additional VAV cooling units can be employed to match cooling requirements.

For maximum energy efficiency and occupant comfort, care should be taken during construction to seal all floor panels. Additional care should be taken to seal all openings through the floor into the space or into walls where plumbing or electrical equipment penetrates the floor plenum. Regular inspection during construction minimizes problems.

Conclusion

In recent years, the application of UFAD systems has shifted from owner-occupied high-tech facilities to a more general variety of building spaces aiming to achieve LEED certification. UFAD provides a high level of comfort by supplying conditioned air where required near an occupant. Additional occupant comfort can be achieved by installing in the core of a building small units with individually adjustable dampers controlled by occupants. By conditioning only the occupied area and stratifying the upper zone with air supplied from the low-pressure floor plenum, energy is saved. Additional energy can be saved by employing a passive VAV cooling and fin-tube heating system on the perimeter.


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Trent Yarbrough is director of engineering for Titus. During his 12 years with the company, he has served as a design engineer, lead engineer, and, most recently, product manager for the terminal-unit, underfloor-air-distribution, and variable-air-volume diffuser groups. He has been a member of ASHRAE for more than 10 years. Jim Aswegan is chief engineer for Titus. He has more than 45 years of experience in the industry. He has served ASHRAE as chair of Technical Committee 5.3, Room Air Distribution; chair of Standard Project Committee 70, Method of Testing for Rating the Performance of Air Outlets and Air Inlets; and a contributor to several Handbook chapters.