Automatic displacement ventilation system with heating mode

A ventilation system includes a supply register with a displacement-type diffuser providing a flow of ventilation air at a non-mixing rate that maintains stratification of air in the occupied space. An first exclusive return register is positioned near the ceiling and a second exclusive return register is positioned near the floor. A control system controls the flow of heated or cooled air to the supply register and selectively controls the flow of air into the first or second return registers. During the cooling mode, cooled air flows through the supply register and air is withdrawn only through the first exclusive return register, while in the heating mode warm air flows through the supply register and air is withdrawn only through the second return register.