Underfloor air distribution

Air returns from the room at ceiling level or the maximum allowable height above the occupied zone.

[1] The UFAD system takes advantage of the thermal plume and stratification phenomenon: the conditioned air is supplied directly to the occupied zone (OZ).

[3] 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 system was introduced into office buildings in the 1970s in West Germany, with the addition of occupant-controlled localized supply diffusers.

UFAD is appropriate for a number of different building types including commercials, schools, churches, airports, museums, libraries, etc.

Other approaches may incorporate fan powered terminal units at the outlets, underfloor ducts, desktop vents or connections to Personal Environmental Control Systems.

[8] Thermal stratification is the result of processes which layer the internal air in accordance with relative density.

[9] UFAD systems capitalize on the natural stratification that occurs when warm air rises due to thermal buoyancy.

The air that the occupant breathes will have a lower concentration of contaminants compared to conventional uniformly mixed systems.

[11] 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.

[12] 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 temperature rise is not significantly affected by the perimeter zone orientation, the internal heat gain and the window-to-wall ratio.

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.

[8] UFAD is particularly suitable for buildings with high height ceilings, where the energy saving effect is more pronounced due to thermal stratification.

[17] Because UFAD is accomplished by supplying air through a raised floor using different types of distribution configurations and outlets, the key issue for efficient performance of the system is to ensure thermal stratification.

[8] 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.

[8] Conventional overhead mixing systems usually locate both the supply and return air ducts at the ceiling level.

[8] Professional and Trade groups that provide research funding and publish standards or guides regarding UFAD systems include:

Diagram of underfloor air distribution showing cool, fresh air moving through the underfloor plenum and supplied via floor diffusers and desktop vents. Warm, stale air is exhausted at the ceiling
Diagram of air movement in an underfloor air distribution system
Air stratification capitalizes on thermal buoyancy to layer high quality supply air at occupant level and leave unoccupied air unconditioned.
Schematic flow diagram of calculation procedure showing transformation from cooling load calculated for an overhead mixing system into a UFAD cooling load, and then divided between the supply plenum, zone (room), and return plenum.
Heat transfer pathways in UFAD system.
UFAD leakage that does not contribute to cooling, leading to wasted increased fan energy.
UFAD leakage into the space, contributing to cooling.