Abstract: A residential dwelling of the type having thermally insulated outer walls and a thermally insulated roof supported by an underlying floor and forming therebetween an essentially closed enclosure, is provided with a tubular vertical stack extending from the floor to the building structure ceiling which spans across the building beneath the roof and separates the attic from the lower occupant rooms. Floor ducts open to the bottom of the stack from the individual rooms and further air ducts within the ceiling open to the top of the stack by way of the attic. A selectively energizable air heating coil is mounted within the stack at its lower end and a selectively energizable air cooling coil is mounted within the stack at its upper end with natural convection effecting air flow through the stack in a given direction depending upon relative energization of the heating or cooling coil.