Underfloor Air Distribution (UFAD) systems provide cooling supply air streams at significantly warmer temperatures than conventional systems, typically 60°F to 68°F. With the use of higher supply air temperatures comes the ability to operate in economizer mode many more hours each year. When producing the higher supply air temperatures, chilled water systems have the ability to operate at much higher chilled water temperatures, thus resulting in a significant increase in the chiller efficiency when producing chilled water. In addition, for systems that will be requiring reheat, additional heating and cooling energy is saved since they will be reheating air that is cooled to only 65°F versus a conventional system that has cooled the air to 55°F.
Since UFAD systems deliver air at lower velocities than conventional system, there is more potential for stratification since room air is mixed less. Thus, a certain portion of the heat in the space will rise towards the ceiling, where it will be exhausted by the return air register. The overall result is that (like with Displacement Ventilation) a portion of the cooling load in the space, including occupant heat gain, lighting and equipment, never appears as a cooling load. Given the fact that at any given point in time, at least some portion of the return air will be exhausted due to outside air requirements, this heat gain will also be exhausted.
Modeling of the UFAD system will follow the same general approach as the Displacement Ventilation systems described earlier. In some instances, fan powered terminal units are used in this application, so a fan powered box may need to be included in the zone inputs. The baseline building does not have an UFAD system.
