Air admitted to the vehicle interior passes through a reactor unit, containing absorption material, at intervals depending upon the temperature of the input air. The energy requirement is calculated by reference to the heat recovery from the vehicle power unit and an electrical heater provides the residual heat required. A fan (2) draws air from the vehicle interior (1) through a duct (3) and outside air through a duct (9) and is controlled by a flap (4) to enter ducts (5) or (10) to be heated by the vehicle water cooling flow or additional heating unit (8). A drying reactor (12) is in the alternative air stream (10') which is controlled by another deflector (11). The air flow may also be directed through a heating unit (17,18). A further deflector (15) directs the dry heated air back to the vehicle interior or the outside. The circulated air may also be cooled (19).
