Under floor air distrbution
Under floor air distribution (UFAD) is a means whereby to distribute the conditioned air from an underfloor supply plenum, created by the slab and access floor, directly to the occupants through various diffusers set-out throughout the space. The UFAD system is similar to that of the overhead (OH) method in terms of the type of equipment used, i.e., air handling units AHU's)
There are several key differences between OH and UFAD - when using a UFAD system you can bring in the supply air at warmer temperatures as it is entering the occupied zone freely, and not mixing with the air that's already in the space. The OH method is essentially a mixing method, whereby air is supplied at the ceiling at higher velocity and at much cooler temperatures than what's desired by the thermostat. The supply air is mixed with the room air to create the uniform temperature desired by the set-points. This method has very low ventilation effectiveness and the contaminant levels within the room are often the same as the air that's being exhausted. Some of the advantages to the UFAD system is the flexibility and sustainability due to the raised floor and fewer interior mechanical requirements, improved personal thermal comfort and lastly improved ventilation effectives (VE). (Ventilation effective is covered in detail below)
Improved thermal comfort is one of the best attributes of a UFAD system, each individual in the space would have his/her own swirl outlet diffuser that would then provide fresh air into the space, thus creating smaller service zones and greater personal control comfort - with the OH system, groups of occupants are sharing one single large ceiling diffuser.
Indoor air quality
Indoor air quality is perhaps the most important aspect when designing a space, the occupants health and safety is critical. Ventilation effectiveness is the measure of how effectively fresh air is introduced into the space. Displacement (UFAD) systems have a higher VE than those of the OH method. VE is represented by the calculation below.
VE = Ce - Cs / Cr - Cs
VE = ventilation effectiveness
Ce = Contaminant levels of exhausted air
Cs = Contaminant levels of supply air
Cr = Contaminant levels of air in the occupied zone
In a perfect OH system the Ce = Cr, therefore providing VE=1.0, we see them ranging closer to 0.8
In a displacement system, Ce>Cr, resulting in VE = 1.2*
Below is a depiction of the OH method vs the UFAD system
*Information gathered from, "MIT's HVAC analysis, June 1st 1999 "The Strata Center" as prepared by Vanderweil Engineers, INC