Concern for indoor air quality continues to have a significant impact on the design and operation of heating, ventilating, and air-conditioning (HVAC) systems. A major player in the performance of such systems is the flow control damper, particularly with the current popularity of the variable air volume type of system. Mixed air damper systems with economizer control have been used for many years to provide energy savings and to provide ventilation air for building occupants. It is often assumed that sufficient mixing of the outdoor air and return air occurs if parallel blade dampers properly aligned are used. However, it has been found that some mixed air dampers do not provide proper mixing of air under cold outdoor air temperature conditions. If the thermal mixing is inadequate to prevent coil freezing, there may also be inadequate mixing of oxygen and gaseous contaminants that could lead to poor indoor air quality (IAQ). This paper includes guidelines for the selection of mixed air damper configurations that provide proper mixing of airstreams. Configurations that do not provide proper mixing are also identified.