In this series of tests, temperature measurements were used to determine the control response and mixing effectiveness of a combination mixing/filter box in a constant-volume air-handling unit. The parallel blade dampers exhibited a control curve with three distinct areas of control response. The first area occurred as the outdoor air damper traveled from the fully closed position to the 15° open position. The second area of control occurred between the outdoor air damper positions of 15° and 60°. The third area of damper control fell between outdoor air damper positions of 60° and 90° (full open). This three-region control curve does not appear to agree with the published control curves for dampers. The mixing effectiveness varied as the damper position changed, and significant stratification existed throughout the range of damper positions tested. The use of parallel blade dampers provided little mixing in this system. In addition, the placement of the outdoor air inlet in the top of the mixing box had little effect upon the mixing effectiveness. The placement of an elbow without turning vanes directly on the outdoor air inlet resulted in the outdoor air being concentrated in the left side of the mixing box. The tests indicated that many factors influence the mixing effectiveness and that more study is needed to determine the relationships that govern the damper control characteristics and the mixing effectiveness of a mixing box.

Units: Dual