Underfloor Objects (UFO) is an air distribution strategy for providing ventilation and space conditioning in buildings as part of the design of an HVAC system. UFO 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. Thermal stratification is one of the featured characteristic of UFO system, which allows higher thermostat setpoints compared to the traditional overhead systems (OH). UFO cooling load profile is different from a traditional OH system due to the impact of raised floor, particularly UFO may has higher peak cooling load than OH systems. UFO has several potential advantages over traditional overhead systems, including layout flexibility, improved thermal comfort, improved ventilation efficiency, improved energy efficiency in suitable climates and reduced 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. UFO is appropriate for a number of different building types including commercials, schools, churches, airports, museums, libraries etc. Notable buildings using UFO system in North America include The New York Times Building, Bank of America Tower and San Francisco Federal Building. Careful considerations need to be paid in the construction phase of UFO systems to ensure a well-sealed plenum to avoid air leakage in UFO supply plenum.
UFO 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, UFO 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 strata 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.
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 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.
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.
Plenum supply air temperature rise is the increase of the conditioned air due to convective heat gain as it travels through the underfloor supply plenum from the plenum inlet to the floor diffusers. This phenomenon is also named thermal decay. Plenum air temperature rise is caused by cool supply air coming into contact with warmer than air concrete slab and raised floor. According to a modeling study, air temperature rise can be quite significant (as much as 5 °C or 9 °F) and subsequently, compared to an idealized simulated UFAD case with no air temperature rise, elevated diffuser air temperatures can lead to higher supply airflow rate and increased fan and chiller energy consumption. The same study found that air temperature rise in summer is higher than in winter and it also depends on the climate. The ground floor with a slab on grade has less temperature rise compared to middle and top floors, and an increase of the supply air temperature causes a decrease in the temperature rise. The temperature rise is not significantly affected by the perimeter zone orientation, the internal heat gain and the window-to-wall ratio. Supply plenum air temperature rise, thus, has implications on the energy saving potential of UFAD systems and their ability to meet cooling requirements with supply temperatures above those of conventional overhead systems. Current research suggests that both energy and thermal performance can be improved in UFAD systems by ducting air to perimeter zones where loads tend to be the greatest. Critics suggest however that such underfloor ducting reduces the benefit of having a low-pressure plenum space, as well as adding design and installation complications when fitting ducts between floor tile pedestals.
Leakage in UFAD supply plenums can be a major cause for inefficiency in a UFAD system. There are two types of leakage—leakage into the space and leakage into pathways that bypass the space. The first category of leakage does not result in an energy penalty because air is getting to the zone it is intended to cool. The second category of leakage increases fan energy in order to maintain a constant plenum pressure, resulting in increased energy use. Careful consideration needs to be paid in the construction phase of UFAD systems to ensure a well-sealed plenum.
The energy assessment of UFAD systems is a not fully solved issue, which has led to numerous research projects within the building science and mechanical engineering community. Proponents of UFAD point to the lower fan pressures required to deliver air in a building via the plenum as compared to through ducts. Typical plenum pressures are 25 pascals (0.0036 psi) (0.1 inch of water column) or less. The improvements in cooling-system efficiency inherent in operation at higher temperatures save energy, and relatively higher supply air temperatures allow longer periods of economizer operation. However, an economizer strategy is highly climate-dependent and necessitates careful control of humidity to avoid condensation. Critics, on the other hand, cite the shortage of rigorous research and testing to account for variations in climate, system design, thermal comfort and air quality to question whether UFAD is able to deliver improved energy efficiency in practice. Limited simulation tools, the shortage of design standards and relatively scarcity of exemplar projects compound these problems.
Underfloor air distribution is frequently used in office buildings, particularly highly-reconfigurable and open plan offices where raised floors are desirable for cable management. UFAD is also common in command centers, IT data centers and Server rooms that have large cooling loads from electronic equipment and requirements for routing power and data cables. The ASHRAE Underfloor Air Distribution Design Guide suggests that any building considering a raised floor for cable distribution should consider UFAD.
Conventional overhead mixing systems usually locate both the supply and return air ducts at the ceiling level. Supply air is supplied at velocities higher than typically acceptable for human comfort and the air temperature may be lower, higher, or the same as desired room temperature depending on the cooling/heating load. High-speed turbulent air jets incoming supply air mix with the room air.
Displacement Ventilation systems (DV) work on similar principals as UFAD systems. DV systems deliver cool air into the conditioned space at or near the floor level and return air at the ceiling level. This works by utilizing the natural buoyancy of warm air and the thermal plumes generated by heat sources as cooler air is delivered from lower elevations. While similar, UFAD tends to encourage more mixing within the occupied zone. The major practical differences are that in UFAD, air is supplied at a higher velocity through smaller-size supply outlets than in DV, and the supply outlets are usually controlled by the occupants.
