TABLE OF CONTENTS This chapter was contributed by Allan Daly.
A commonly cited benefit of UFAD systems is that they save energy when compared to "standard" overhead (OH) air distribution systems. At present, this claim is difficult to prove quantitatively because of the lack of an energy-modeling tool that properly addresses all of the issues related to energy use with underfloor systems that would allow direct comparison of two simulated systems.
To help designers understand the energy impacts of UFAD, this section explains the variety of factors that affect energy use in a descriptive manner. Topics include air distribution, economizer operation, cooling system efficiency, occupant thermal comfort, and pre-cooling strategies.
As described in Chapter 4, the underfloor plenum is a primary air distribution route. Because the use of the plenum eliminates the need for some portion of the ductwork, and because the large size of the plenum creates little restriction to the flow of air, the amount of fan pressure required to deliver air in a building using UFAD can be less than that required in an equivalent OH system.
For example, in an OH VAV-reheat system, a typical central fan design might provide 3 in. H 2O (750 Pa) of pressure to move air through the index run. Typically to 1 in. (125 250 Pa) of this pressure might be required by the VAV box, reheat coil, and downstream low-pressure ductwork to the diffuser. Depending on the specific implementation of an equivalent UFAD system, most of this...
