TABLE OF CONTENTS The movement of air through the conditioned spaces of buildings plays a critical role in the performance of a building's heating, ventilating, and air-conditioning (HVAC) system, directly affecting thermal comfort, indoor air quality, and energy use. Most of the potential performance advantages of underfloor air distribution (UFAD) systems over conventional air distribution system design arise from the fact that conditioned air is delivered at or near floor level, directly into the occupied zone of the building, and is returned at or near ceiling level. In this chapter, three approaches to room air distribution design (overhead mixing, displacement ventilation, UFAD) are described and compared to illustrate the characteristics of room air distribution using UFAD systems.
Historically, the approach to HVAC design in commercial buildings has been to both supply and remove air at ceiling level (Figure 2.1). Conditioned air is typically supplied at velocities that are much higher than those acceptable for occupant comfort. Supply air temperature may be lower, higher, or equal to the desired room air temperature set-point, depending on the cooling/heating load. Incoming high-speed turbulent air jets create rapid mixing with the room air so that the supply jet's temperature quickly approaches that of the entire room. As the jet proceeds into the room, it entrains room (secondary) air into the primary air jet, causing it to grow and spread in size and therefore to reduce in air speed. A system of overhead diffusers is designed and operated so that the ceiling-based supply...
