Abstract: Air mixing apparatus that is particularly useful in connection with a winter make-up air and a summer ventilation system for a factory, warehouse or the like includes inner and outer relatively movable ducts, the former having side walls, an air inlet through which outside air can be drawn into the outer duct and an air outlet through which air can exit from the outer duct. At least one first port is provided in the side walls of the outer duct for passage of inside air, i.e, air from inside the building, into the outer duct. The inner duct has at least one second port in its side walls through which air can pass into the inner duct, and the inner duct also has an air outlet for passage of air into the outer duct where it can exit from the air outlet of the outer duct. Baffles extend between the inner and outer ducts on either side of the at least one first port and with the inner and outer ducts define a header. A motor or the like can move the ducts relative to each other between two positions.