Designers have considerable flexibility in selecting the supply air temperature and corresponding air quantity within the limitations of the procedures for determining heating and cooling loads. ASHRAE Standard 55 also addresses the effect of these variables on comfort. In establishing the supply air temperature, the initial cost of lower airflow and low air temperature (smaller fan and duct systems) must be calculated against the potential problems of distribution, condensation, air movement, and the presence of increased odors and gaseous or particulate contaminants. Terminal devices that use low-temperature air can reduce the air distribution cost. These devices mix room and primary air to maintain reasonable air movement in the occupied space. Because the amount of outside air needed is the same for any system, the percentage in low temperature systems is high, requiring special care in design to avoid freezing of preheat or cooling coils. Also, the low-temperature air supply reduces humidity in the space. Lower humidity during cooling cycles costs more in energy because the equipment runs longer. Also, if the humidity is too low, it may cause respiratory problems.
