Underfloor air distribution

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.

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.

UFAD cooling load

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.

UFAD design tools for zone airflow requirements

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.

ASHRAE Research Project (RP-1522) developed a simplified tool that predicts the vertical temperature difference between the head and ankle of occupants, the supply air flow rate for one plenum zone, number of diffusers and the air distribution effectiveness. The tool requires users to specify the zone cooling load and the fraction of the cooling load assigned to the underfloor plenum. It also requires users to input the supply air temperature either at the diffuser or at the duct but with the ratio of plenum flowrate to zonal supply flowrate. The tool allows users to select from three type of diffusers and is applicable to seven type of buildings, including office, classroom, workshop, restaurant, retail shop, conference room and auditorium.

The CBE UFAD design tool based on extensive research is able to predict the cooling load for UFAD system with the input of the design cooling load calculated for the same building with an overhead system. It also predicts the airflow rate, room temperature stratification, and the plenum temperature gain for both interior and perimeter zones of a typical multi-story office buildings using UFAD system. The CBE tool allows the user to select from four different plenum configurations (series, reverse series, independent and common) and three floor-diffusers (swirl, square and linear bar grill). An online version of the design tool is publicly available at Center for the Built Environment.

Plenum air temperature rise

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.

Air leakage in UFAD plenums

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.

UFAD and energy

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.

Applications

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.

Specific space considerations should be taken when using UFAD systems in laboratories because of its critical room pressurization requirements and potential migration of chemicals into the access floor plenum due to spillage. UFAD systems are not recommended in some specific facilities or spaces, such as small non-residential buildings, wet spaces like restrooms and pool areas, kitchens and dining areas and gymnasiums, because UFAD may result in especially difficult or costly in design. UFAD systems may also be used with other HVAC systems, like displacement ventilation, overhead air distribution systems, radiant ceiling or chilled beam systems to get better performance.

Overhead (mixing)

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.

A well-engineered UFAD systems have several potential advantages over traditional overhead systems, such as layout flexibility, improved thermal comfort, improved ventilation efficiency and indoor air quality, improved energy efficiency in suitable climates and reduced life cycle costs.

Displacement ventilation

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.