TABLE OF CONTENTS Due to the reduction in supply ductwork and to the low operational static pressures in pressurized underfloor air supply plenums (typical pressures are around 0.05 0.1 in. H 2O [12.5 25 Pa]), central fan energy use and installed horsepower can potentially be reduced relative to traditional ducted overhead air distribution systems (see Webster et al. [2000] for a more complete discussion of this issue). Similarly, in zero-pressure plenums, fan-assisted supply outlets further reduce central fan sizing. As discussed in Section 12.7, cooling air quantities, and therefore air-handler capacities, for UFAD systems should be carefully determined and may be equal to or less than those required under the same conditions using overhead systems.
Humidity control is a key consideration in the selection of air-handling units (AHU) and the mechanical cooling plant. In all climates requiring dehumidification of the outside air, a cooling coil leaving temperature in the range of 50 55 F (10 13 C) will typically be used. To produce the higher plenum inlet temperatures (61 65 F [16 18 C]) required for UFAD systems, a non-standard AHU configuration, such as face and bypass, must be specified. With this approach, the incoming outside air and a portion of the return air are dehumidified (minimum amount needed for humidity control). The remaining return air is bypassed around the coil, if done at the air handler, and mixed with the cool primary air to produce supply air of the proper temperature and humidity before being delivered directly into the underfloor plenum. In this configuration,...
