Apparatus for blending and destratification of outside and return air streams which are cooled or heated, as by fluid containing heat transfer coils, and discharged into a building. Gates control the flow and blending of outside and inside air in a casing in a region defined at its downstream end by the coils and at its upstream end by the gates which project into the region. The gates are pivotally mounted and vary the volume in the region, and also increase the velocity of air therein to prevent stratification. An array of sensors monitors the temperature of the air passing through the coils and controls the gates, especially when the velocity is low, thereby preventing any freezing of the fluid in the coils which can cause damage. The gates may be panels filled with insulation which provide a seal to protect the coils when the apparatus is not in use. A programmed computer receives inputs from the temperature sensor array and from a discharge air velocity sensor for independently actuating the gates to effect blending, and effect destratification, when destratification is necessary.