Underfloor air distribution

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 through floor diffusers directly into the occupied zone of the building. UFAD systems are similar to conventional overhead systems (OH) in terms of the types of equipment used at the cooling and heating plants and primary air-handling units (AHU). Key differences include the use of an underfloor air supply plenum, warmer supply air temperatures, localized air distribution (with or without individual control) and thermal stratification. Thermal stratification is one of the featured characteristics of UFAD systems, which allows higher thermostat setpoints compared to the traditional overhead systems (OH). UFAD cooling load profile is different from a traditional OH system due to the impact of raised floor, particularly UFAD may have a higher peak cooling load than that of OH systems. This is because heat is gained from building penetrations and gaps within the structure itself.UFAD has several potential advantages over traditional overhead systems, including layout flexibility, improved thermal comfort and ventilation efficiency, reduced energy use in suitable climates and life-cycle costs. 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 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. 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 through floor diffusers directly into the occupied zone of the building. UFAD systems are similar to conventional overhead systems (OH) in terms of the types of equipment used at the cooling and heating plants and primary air-handling units (AHU). Key differences include the use of an underfloor air supply plenum, warmer supply air temperatures, localized air distribution (with or without individual control) and thermal stratification. Thermal stratification is one of the featured characteristics of UFAD systems, which allows higher thermostat setpoints compared to the traditional overhead systems (OH). UFAD cooling load profile is different from a traditional OH system due to the impact of raised floor, particularly UFAD may have a higher peak cooling load than that of OH systems. This is because heat is gained from building penetrations and gaps within the structure itself.UFAD has several potential advantages over traditional overhead systems, including layout flexibility, improved thermal comfort and ventilation efficiency, reduced energy use in suitable climates and life-cycle costs. 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 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. 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. 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. Cooling load profiles for UFAD systems and overhead systems are different, mainly due to the thermal storage effect of the lighter-weight raised floor panels compared to the heavier mass of a structural floor slab. The mere presence of the raised floor reduces the ability of the slab to store heat, thereby producing for the system with a raised floor higher peak cooling loads compared to the system without a raised floor. In the OH system, particularly in perimeter zones, part of the incoming solar heat gain is stored in the floor slab during the day, thus reducing peak zone cooling loads, and released at night when the system is off. In a UFAD system, the presence of the raised flooring transforms the solar absorbing massive floor slab into a lighter weight material, leading to relatively higher peak zone cooling loads. A modeling study based on EnergyPlus simulations showed that, generally, UFAD has a peak cooling load 19% higher than an overhead cooling load and 22% and 37% of the total zone UFAD cooling load goes to the supply plenum in the perimeter and interior, respectively. Center for the Built Environment developed a new index UFAD cooling load ratio (UCLR), which is defined by the ratio of the peak cooling load calculated for UFAD to the peak cooling load calculated for a well-mixed system, to calculate the UFAD cooling load for each zone with the traditional peak cooling load of an overhead (well-mixed) system. UCLR is determined by zone type, floor level and the zone orientation. The Supply Plenum Fraction (SPF), Zone Fraction (ZF) and Return Plenum Fraction (RPF) are developed similarly to calculate the supply plenum, zone and return plenum cooling load. There are two available design tools for determining zone airflow rate requirements for UFAD system, one is developed at Purdue University as part of the ASHRAE Research Project (RP-1522). The other one is developed at Center for the Built Environment (CBE) at University of California Berkeley.