A temperature-control system for a vehicular passage compartment having a pair of sides is provided with a housing forming an elongated mixing chamber having a fresh-air inlet slot and a modified-air inlet slot each extending substantially the full length of the chamber which also has a pair of outlets each at a respective longitudinal end of the chamber. Fresh air is fed to the chamber through the fresh-air inlet slot and either heated or cooled air is fed to the chamber through the modified-air inlet slot. A conduit leads from each of the outlets to a respective side of the motor-vehicle compartment so that a mixture of temperature-modified and fresh air can be fed to the two sides of the passenger compartment. A deflector is longitudinally displaceable in the chamber along one of the inlet slots, normally the modified-air inlet slot, and partially blocks this one slot in any position therealong. Thus displacement of the deflector can vary the mix ratio of fresh air and modified air in that portion of the mixing compartment to vary the heating or cooling effect between the two sides of the motor-vehicle passenger compartment.