Underfloor Air Distribution (UFAD) by Asif Syed

Underfloor Air Distribution (UFAD) is a method of air conditioning the space by supplying the air from the floor, using natural buoyancy forces to lift it to the ceiling, as opposed to the conventional systems, which supply air from the ceiling down to the occupants, working against the natural forces of buoyancy.

The advantages of the system include energy efficiency, thermal comfort, individual occupant control, flexibility for frequent office restructuring, better indoor air quality, and lower costs for churn fit-out.

UFAD technology uses an air plenum under the floor to supply air from floor air outlets. An air plenum is made between the floor and the structural slab. This requires a raised floor plenum of sufficient depth to transport the air from the supply source to the air outlets. The plenum space is easily accessible and provides the same level of access as a two-foot-by-two-foot (60-centimeter-by-60-centimeter) ceiling tile — without the necessity of climbing a ladder.

The space under the raised floor, primarily created for air flow, is also used for the distribution of other services, such as electrical power wiring, telephone and information technology cabling, security cabling, and fire alarms.

The use of a raised-floor plenum for other services makes the system flexible to modifications, due to ease of access. Given the dependence of business on computers, networks, VOIP, and so on, and the fast-paced changes in technology, the demand for flexibility in data cabling is ever increasing.

The environmental and energy benefits come from the operating temperatures, which are much higher (in cooling applications) than in conventional systems. The combination of environmental and energy benefits with flexibility is the main reason for the growing popularity of the systems. The technology is not new to buildings; from the 1950s on, it has been used in data centers or computer rooms. The driver then was flexible cable management and efficient cooling of high heat loads. Today the drivers are: energy conservation, the environment, and flexibility in managing other building services.

The LEED 2009 Reference Guide for Green Building Design and Construction cites UFAD as a way to achieve individual occupant controls. From the 1970s, buildings have been designed with this technology in Europe and Asia, and have worked satisfactorily.

In the United States, UFAD technology was introduced in the 1990s, and several buildings have been designed to incorporate it.

Initially, designs in the United States had mixed results. Projects where design and construction techniques were not adapted to the technology had poor results and generated bad press for the technology. Projects where the designers and contractors tapped into European experience and expertise had very successful outcomes.

However, the U.S. building industry has learned the lessons of implementing the new technology and lately has built high-profile successful projects. Initial construction costs associated with UFAD systems are slightly higher than those of conventional systems, but the overall life-cycle costs are far less.

Three major new projects in New York City, with approximately 5 million square feet of construction, have been built in the last five years using UFAD technology. The New York Times headquarters building uses UFAD for 800,000 square feet of office space. The One Bryant Park building uses UFAD in addition to thermal storage, high-performance envelope, and daylight harvesting as steps toward achieving LEED Silver certification. Another major corporation in New York (not named for confidentiality) used UFAD in their new headquarters building.

Benefits of UFAD

The systematic benefits of UFAD, applied in appropriate building types, include energy efficiency, thermal comfort and individual occupant control, indoor air quality, low noise levels, and flexibility, in terms of rearranging office layouts economically.

Looking at energy efficiency factos in more detail:

1. Reduced energy consumption in UFAD systems comes from:
1. Higher air temperature: The supply air temperature of the UFAD system is higher than that of a conventional overhead air distribution system. The UFAD supply air temperature is 65 degrees Farenheit (18.3 degrees Celsius), whereas conventional air systems use 55 degrees Farenheit (12.8 degrees Celsius) air. The benefit from higher temperature is twofold:
   1. The cooling system or the compressors will not have to work as hard, and will therefore use less energy. The refrigeration process of absorbing heat from the lower-temperature source (indoors) and rejecting heat to the high-temperature sink (outdoors) is more efficient when the source is at a higher temperature.
   2. Economizer or free cooling is used in the air conditioning system. This is a method that uses outdoor ambient cooler conditions to cool an indoor heat-generating space. Office interior spaces require cooling even in the winter, when outdoor temperatures are cold. It is only the perimeter building envelope space that requires heat, as a result of skin loss. For UFAD systems, the economizer period can be extended. In conventional systems, economizer or free cooling stops when the outdoor temperature reaches 55 degrees Farenheit (12.8 degrees Celsius). In UFAD systems, the free cooling can continue up to 65 degrees Farenheit (18.3 degrees Celsius) (with acceptable relative humidity).
2. Thermal stratification: UFAD systems stratify the space above the head level up to the underside of the ceiling with higher temperatures — 80 to 85 degrees Farenheit (26.7 to 29.4 degrees Celsius). From the floor to just above head level is considered the occupied zone and is maintained at comfort temperature — 72 to 78 degrees Farenheit (22.2 to 25.6 degrees Celsius). Conventional systems maintain a uniform comfort temperature — 72 to 78 degrees Farenheit (22.2 to 25.6 degrees Celsius) over the entire height of the occupied space.In a space with 10-foot (three-meter) ceilings, about six feet (1.8 meters) is the occupied zone, and four feet (1.2 meters) is the unoccupied zone. Thus, UFAD systems condition only 60 percent of the volume of space to human comfort, and the remaining 40 percent is above the comfort zone. This process of stratification permits most of the heat from the lights, some of the solar heat gain from glazing, and a portion of the envelope heat gain to be returned directly to the cooling system, without any impact on the occupied zone of the space.
3. Low fan energy: The air quantity required is the same for both conventional and UFAD systems. However, UFAD systems distribute air through a plenum with less air pressure loss compared to a conventional system with ductwork and variable air volume (VAV) boxes (which are dampers or valves or gates to control flow). The lower pressure loss results in lower
   fan energy consumption. Some conventional systems employ fans in the ceilings, such as in the fan-powered VAV systems; the energy consumed by these fans is also eliminated.
4. Diurnal temperature advantage: Diurnal temperature is the daily fluctuation in temperature between night and day. The lower temperature in the nights, when the building is not occupied, can be used to cool the building mass, for example the floor slabs. The air is directly in contact with the mass of the structural floor slabs. Conventional systems do not permit this. A night purge cycle can cool the building and increase the thermal inertia, which can be used during the daytime, reducing energy use and peak demand. Asif Syed, PE, LEED-AP, is an engineer and is currently a partner at AKF Engineers, where he is responsible for design and analysis of building mechanical systems..

For more info please click here