Abstract: A vehicle heating and cooling system has an air duct arrangement that makes efficient use of the available duct space. The initial duct in the system is forced to run through a body area of limited size, and so has a limited cross sectional area and consequently limited air handling capacity. It also contains side window defroster vents that should not be allowed to vent cold air. Therefore, it would be normally not be considered possible to run both hot and cold air through the duct. The arrangement provides a sealed, expansible and collapsible sleeve running through the rigid duct, which expands full size when cold air is fed into it, and which collapses almost completely when hot air is forced past it and through the rigid duct only. Therefore, both hot and cold air can be run through the same volume, and cold air is kept away from the defroster vents.