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Last edited 19 Jan 2021
Underfloor air distribution UFAD
Underfloor air distribution (UFAD) is a strategy for providing ventilation and space conditioning in buildings as part of an HVAC system. UFAD systems use the underfloor plenum beneath a raised floor to provide conditioned air through floor diffusers directly to the occupied zone.
Once the air enters through the floor diffusers, it spreads across the floor forming a reservoir of fresh cool air. Any sources of heat (such as people or computers) generate a thermal plume lifting contaminated air to high level for removal from the space.
Since air is supplied in much closer proximity to the occupants than in conventional overhead systems, supply air temperatures must be higher. Usually, a task/ambient conditioning (TAC) system is used to allow individuals to control thermal conditions in small and localised zones.
Under floor air distribution is frequently used in office buildings, particularly highly-reconfigurable and open plan offices with raised floors. UFAD is also common in command centres, IT data centres and server rooms that have large cooling loads from electronic equipment and requirements for routing power and data cables. The ASHRAE Under floor Air Distribution Design Guide suggests that any building considering a raised floor for cable distribution should consider UFAD.
Currently, three types of UFAD systems are available:
- Supply air delivered via passive floor registers and/or fan-powered terminal boxes supplied by a pressurised underfloor plenum and central air handler.
- Supply air delivered via active, locally-controlled, fan-powered registers (in the floor or workstations), supplied by a very low-pressure underfloor plenum and central air handler.
- Supply air delivered via under floor ducts to terminal devices or supply outlets.
- Improved thermal comfort by allowing individual occupants to control their local thermal environment.
- Improved ventilation efficiency and indoor air quality by delivering fresh supply air at floor level or near the occupant.
- Reduced energy use: Cooling energy savings from economiser operation and increased chiller COP (coefficient of performance). Fan energy savings.
- Significantly reduced life-cycle building costs due to reduced expenses associated with occupant "churn," and remodelling or changing the interior.
- Reduced floor-to-floor height in new construction by reducing the overall height of the service plenum.
- Improved productivity and health.
- New and unfamiliar technology: Lack of familiarity can create problems throughout the building design, construction, and operation process, including higher cost estimates, incompatible construction methods, and incorrect building control and operation on the part of both facility managers and building occupants.
- Lack of information and design guidelines.
- Difficulties modelling whole-building performance.
- Higher initial costs.
- Cold feet and draught discomfort: Poorly designed (e.g., the close proximity of supply outlets to the occupants) and operated UFAD system can cause cold floor problems. To prevent this, all office floors are recommended to be carpeted.
- Condensation and dehumidification issue: In humid climates, outside air must be properly dehumidified before delivering supply air to the underfloor plenum.
- Spillage and dirt entering UFAD systems: There is the possibility of spillage and dirt entering directly into the underfloor supply airstream. Most floor diffusers, however, have been designed with catch-basins to hold the liquid from a typical soft drink spill.
- Limited availability of UFAD products.
 UFAD System Design Process
- Initial building design considerations.
- Select system configuration.
- Determine space cooling and heating loads.
- Determine ventilation air requirements.
- Determine zone supply air temperature and flow rate.
- Determine return air configuration.
- Calculate cooling coil load.
- Layout ducts and plenum configuration.
- Select primary HVAC equipment.
- Select and locate diffusers.
- Develop a control strategy.
There are several approaches to address zones with significantly different thermal loads:
- Plenum partitioning with ducted VAV (Variable Air Volume) devices supplying air to each zone.
- Plenum partitioning with fan-powered terminal devices supplying air to each zone.
- Thermostatically controlled VAV diffusers may be used in both partitioned and open plenums.
- Local fan-driven supply outlets may be used in both partitioned and open plenums.
- Open plenums with mixing boxes and ducted outlets.
 Supply air temperature and flow rate
Because the air is supplied directly into the occupied zone, supply air temperatures must be warmer than for conventional overhead system design. For cooling applications, supply air temperatures at the diffusers should be maintained no lower than 17 – 20°C (63 – 68°F) to avoid overcooling nearby occupants. This supply temperature can be even higher under partial load conditions.
Mixed air temperature after the cooling coil, or plenum inlet temperature must be determined by taking into account temperature increase (or decrease, depending on the slab temperature) as the air flows through the underfloor plenum. Current estimates for typical air flow rates in an underfloor plenum with a slab that is 3°C (5°F) warmer than the plenum inlet air temperature call for a 1°C (2°F) increase for every 10 m (33 ft) of distance travelled through the plenum.
In temperate climates, where high humidity is not a problem, these warmer supply air temperatures increase the potential for economiser use, and allow higher cooling coil temperatures to be set, if desired.
Cooling air quantities for UFAD systems should be carefully determined. Higher supply air temperatures would suggest that higher supply air volumes are required, but the higher return temperatures created by stratification reduce the required increase in volume. A calculation of the portion of heat sources that bypass the occupied zone in the space allows cooling air quantities to be further reduced. The net effect is that for most designs, controlled stratification in the space allows cooling air quantities for UFAD systems to be equal to or less than those required under the same conditions using overhead air distribution.
With UFAD systems, conditioned air from the air handling unit (AHU) is ducted into the underfloor plenum where it typically flows freely to the supply outlets. Underfloor systems are generally configured to have a relatively large number of smaller supply outlets, many in close proximity to the building occupants, as compared to a conventional overhead system.
The flexibility of mounting supply diffusers in movable raised access floor panels is a major advantage of UFAD systems. The inherent ability to easily move diffusers to more closely match the distribution of loads in the space makes the placement of diffusers a much easier task. In fact, initial layouts can be quite crude. Final placement can take place after the location of furniture and loads and when the preferences of individual occupants have been more accurately determined.
Passive diffusers can generally be converted to an active diffuser by simply attaching a fan-powered outlet box to the underside of the diffuser or grill.
 Construction Phase Guidelines
- It is essential that the implications of the raised access floor be considered early in the design process.
- The concrete slab surface must be sealed to reduce dust, and the underfloor plenum and floor panels must be thoroughly cleaned both during installation of the access floor and again before occupancy.
- The height of the access floor and the placement of the 0.6 m x 0.6 m (2 ft x 2 ft) raised floor pedestal grid is critical with respect to locating all underfloor service installations.
- It is important to layout underfloor equipment requiring regular maintenance in accessible areas, such as corridors, not underneath furniture and partitions.
- In partitioned office spaces, offset the partition grid from the floor grid so that partitions do not cover joints between floor panels, thereby preventing access to the underfloor plenum on both sides of the partition.
- Consider dead load allowance and seismic bracing of the access floor.
- Determine areas in the building with no access floor and allow for transitions to areas with access flooring.
- In pressurised underfloor air distribution systems, greater care must be taken during construction to seal the underfloor plenum to prevent uncontrolled air leakage.
- Designers must consider that fan rooms or access for HVAC distribution will be required at more frequent intervals than with conventional air distribution systems.
- If called for, return air shafts must be designed between the ceiling and the underfloor plenum, usually around columns or other permanent building elements.
- The main structural slab, the traditional working platform, will not be available continuously during construction, and therefore a well coordinated construction sequence is necessary.
 Find out more
 Related articles on Designing Buildings Wiki
- Displacement ventilation.
- Dynamic thermal modelling of closed loop geothermal heat pump systems.
- Earth-to-air heat exchangers.
- Floating floor.
- Geothermal energy.
- Geothermal pile foundations.
- Ground energy options
- Ground preconditioning of supply air.
- Ground source heat pumps.
- Raised floor.
- Thermal comfort.
- Thermal labyrinths.
- Underfloor air conditioning at London Grade II listed landmark.
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