Abstract: A heating, ventilating and air conditioning system for vehicles incorporating a rotary air valve mounted in a housing for limited arcuate movement with respect to a pivot axis to adjust the amount of opening of two adjacent air flow windows to thereby control the routing and quantity of air flow through the heater core and a heater core bypass. The rotary valve is moved in a track radially spaced above peripheral edges in the housing defining the windows to eliminate frictional drag and wear of the sealing face of the air valve as well as the load on the motor or mechanical actuator. The valve is positioned at an infinitely variable number of positions for partial opening of the windows and a camming system is used to move the valve without drag into or from sealing positions with respect to the two windows.