Underfloor Air Distribution (UFAD) is an air distribution strategy for providing ventilation and space conditioning in buildings as part of the design of an HVAC system. UFAD systems use the underflow plenum beneath a raised floor to provide conditioned air through floor diffusers directly to the occupied zone. The concept of underfloor air is not new, however, changes in office space usage, sustainable design and indoor air quality issues have sparked considerable recent interest in the concept. Underfloor air conditioning had its start in computer rooms, at a time when mainframe computers generated considerable heat and had a considerable amount of complex cabling required by the computers. Access floor systems allowed plenty of open space to run cabling and a generous pathway to supply large quantities of cooling air under the intense heat of the electronics. The natural convection currents of warm air rising allowed cool air to enter low, cool the equipment and remove the warm air near the ceiling.

Below are the salient features of the underfloor air distribution system:

Reduced Lifecycle Cost

The large open floor plenum was a convenient space to run large amounts of cable for power and communications. The greatest change in offices in the last 15 years has been an ever-growing need for voice, data and power connections to every worker’s workstation. This coupled with one other trend in office space design that is the use of more open plan space and a trend to move workers into cross-functional workgroups with great regularity.

Improved Comfort

The second feature of underfloor air distribution (UFAD) was that the same principle of warm air rising from hot electrical gear could also be applied to warm air around people. Traditional overhead air distribution systems are designed to do a very good job of mixing the air in the space and prevent stratification in the room. Without increased complexity and cost delivering thermal comfort to every occupant while also providing personal ventilation may be cost prohibitive. Overhead systems may require more fan energy to overcome static losses of generating the mixing, airflow patterns required within the space.

By applying the warm air rising principle, air can be provided below the occupants and discharged directly into the breathing zone at relatively low velocity. The people warm the air and by natural convection the air rises toward the ceiling. Since people only breath air in a zone from approximately the floor to six feet, the space above this zone can be treated as a stratified air layer and the load components in this zone treated differently. The result is that air provided underfloor can be supplied at low pressure and the energy for space conditioning can be reduced.

Other Advantages

I. In addition to being either partitioned or open-plan, plenums can operate as pressurized or zero-pressure. Pressurized plenums have a small positive static pressure applied, created by a central fan in the air-handling unit (AHU), that drives the air up through the diffusers to the lower pressure surroundings of the conditioned office space. Standard methods of control at the central AHU are used to maintain the desired overall supply volume and temperature under thermostatic control, and a pre-set pressure level, at which the diffusers achieve optimum performance. The low operational pressure of underfloor plenums, compared to ceiling-based systems, potentially reduces fan energy consumption and thus, operating costs.
II. In zero-pressure plenums, small local fan units draw air from the underfloor plenum and deliver it to the space through floor diffusers or outlets located in the furniture or partitions. Compared to passive diffusers, used with pressurized-plenum systems, fan-powered outlets are generally more easily and effectively controlled by nearby occupants. However, for reasons of reduced maintenance, installation and equipment costs, pressurized-plenums are the most commonly installed systems at present.


UFAD is a widely accepted concept of air distribution globally and have various advantages compared with overhead air distribution system. There might be slightly more initial cost but the running cost is low and has a return of investment of three to four years.