TABLE OF CONTENTS For optimal cooling operation of a UFAD system, it is important to locate return grilles at ceiling level or, at a minimum, above the occupied zone (1.8 m [6 ft]). Air is typically returned through grilles located in a suspended ceiling or through high side-wall grilles if no ceiling plenum is present. This supports an overall floor-to-ceiling air flow pattern that takes advantage of the natural buoyancy produced by heat sources in the office and more efficiently removes heat loads and contaminants from the space.
A certain portion of return air is mixed with primary air from the AHU to achieve desired air temperatures and humidity and enable reduced energy costs. In many climates, to achieve proper humidity control, conventional cooling coil temperatures must be used (producing a coil leaving temperature of 55 F [13 C]). In this situation, a return air bypass control strategy can be employed in which a portion of the return air is bypassed around the cooling coil and then mixed with the air leaving the coil to produce the desired warmer supply air temperature entering the plenum (61 65 F [16 18 C]).
In some cases, a percentage of return air can be recirculated directly back into the underfloor plenum via return shafts near the ceiling or from the ceiling plenum. As mentioned earlier, this configuration does not allow filtering of return air back at the AHU. Room air flowing back into the plenum through open floor grilles can also serve as makeup air for zero...
