Suhail (1989) points out that, "Laboratories, research facilities, clean rooms, and hospitals require large amounts of outside make-up air. These facilities can only benefit from economizers when the facility does not require 100 percent outside make-up air. Exhaust air from the facility may be added into this outside make-up air stream with an economizer to save energy. Inadequate mixing of the make-up air stream and the exhaust (return) air will defeat the function of economizer." Consequently, errors made by the mixed-air temperature sensor "can increase annual energy consumption by as much as 30 to 50 percent." In the same report, Suhail (1989) describes a study by Kao et al.of economizer operation, which found that "a sensor error of only 1°F wasted 10 percent of the cooling energy and 6 percent of the heating energy in the HVAC systems studied." To maximize the savings from economizer system, proper blending and sensing of the air streams must occur. Stratified, unblended joining of the return and outside air streams in HVAC economizer systems can result in varied building temperatures, uneven filter distribution, and, in the right climate, frozen coils. [Suhail, 1989]
