Underfloor air distribution (UFAD) is an air distribution strategy for providing ventilation and space conditioning in buildings as part of the design of a HVAC system. UFAD systems use an underfloor supply plenum located between the structural concrete slab and a raised floor system to supply conditioned air to supply outlets (usually floor diffusers), located at or near floor level within the occupied space. Air returns from the room at ceiling level or the maximum allowable height above the occupied zone. The UFAD system takes advantage of the thermal plume and stratification phenomenon: the conditioned air is supplied directly to the occupied zone (OZ). The thermal plumes generated by the occupants and other heat sources introduce the conditioned air to absorb the heat and humidity and then bring the contaminated air to the upper zone (UZ). At a certain plane in the room, the airflow rate returned to the upper zone is equal to the supply air. The plane divides the room into occupied zone and upper zone and leads to thermal stratification: the hot and contaminated air is concentrated in the upper zone, and the air in the occupied zone is cool and fresh. UFAD can bring several potential advantages over traditional overhead systems, including reduced life-cycle building costs; improved thermal comfort, occupant satisfaction, and productivity; improved ventilation efficiency, indoor air quality, and health; reduced energy use and static pressures; and reduced floor-to-floor height in new construction. An under-floor air distribution concept combined with a ceiling-distributed returns ventilation layout (UFAD-CDR) can dramatically reduce the risk of airborne transmission at both high and low ACHs. The UFAD system was originally introduced in the 1950s for rooms with high heat loads and raised floors systems for cable and equipment management (e.g. computer rooms, control centers, etc.). The system was introduced into office buildings in the 1970s in West Germany, with the addition of occupant-controlled localized supply diffusers. Nowadays UFAD system has achieved considerable acceptance in Europe, South Africa, and Japan. UFAD is often used in office buildings, particularly highly-reconfigurable and open plan offices where raised floors are desirable for cable management. UFAD is appropriate for a number of different building types including commercials, schools, churches, airports, museums, libraries, etc. Notable buildings using the UFAD system in North America include The New York Times Building, Bank of America Tower and San Francisco Federal Building. Careful considerations need to be made in the construction phase of UFAD systems to ensure a well-sealed plenum to avoid air leakage in UFAD supply plenums.
System description UFAD systems rely on air handling units to filter and condition air to the appropriate supply conditions so it can be delivered to the occupied zone. While overhead systems typically use ducts to distribute the air, UFAD systems use the underfloor plenum formed by installation of a raised floor. The plenum generally sits 0.3 and 0.46 metres (12 and 18 in) above the structural concrete slab, although lower heights are possible. Specially designed floor diffusers are used as the supply outlets. The most common UFAD configuration consists of a central air handling unit delivering air through a pressurized plenum and into the space through floor diffusers. Other approaches may incorporate fan powered terminal units at the outlets, underfloor ducts, desktop vents or connections to Personal Environmental Control Systems.
UFAD air distribution and stratification Thermal stratification is the result of processes which layer the internal air in accordance with relative density. The resulting air stratum is a vertical gradient with high-density and cooler air below and low-density and warmer air above. Due to the naturally convective movement of air, stratification is used predominantly in cooling conditions.
UFAD systems capitalize on the natural stratification that occurs when warm air rises due to thermal buoyancy. In a UFAD design, conditioned air stays in the lower, occupied part of the room, while heat sources such as occupants and equipment generate thermal plumes, which carry the warm air and heat source generated pollutants towards the ceiling where they are exhausted through the return air ducts. The temperature stratification created by the UFAD system has implication for space setpoints. Most of an occupant's body is in an area that is colder than the temperature at the thermostat height; therefore, current practice recommends raising thermostat setpoints compared to traditional overhead systems. The optimal ventilation strategy controls the supply outlets to limit the mixing of supply air with room air to just below the breathing height of the space. Above this height, stratified and more polluted air is allowed to occur. The air that the occupant breathes will have a lower concentration of contaminants compared to conventional uniformly mixed systems. The theoretical behavior of UFAD systems is based on the plume theory for DV systems. In comparison to classic displacement ventilation (DV) systems that deliver air at low velocities, typical UFAD systems deliver air through floor diffusers with higher supply air velocities. In addition to increasing the amount of mixing (and therefore potentially diminishing the ventilation performance compared to DV systems), these more powerful supply air conditions can have significant impacts on room air stratification and thermal comfort in the occupied zone. Therefore, the control and optimization of this stratification is crucial to system design and sizing, energy-efficient operation, and comfort performance of UFAD systems. Many factors, including the ceiling height, diffuser characteristics, number of diffusers, supply air temperature, total flow rate, cooling load and conditioning mode affect the ventilation efficiency of UFAD systems. Swirl and perforated-floor-panel diffusers have been shown to create a low air velocity in the occupied zone, while linear diffusers created the highest velocity in the occupied zone, disturbing thermal stratification and posing a potential draft risk. Additionally, floor diffusers add an element of personal control within the reach of the occupant, as users can adjust the amount of air that is delivered by the diffuser though rotating the diffuser top.
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