The device has an air guide channel (30) provided between air inlet and outlet openings in a flow direction. A heat exchanger (20) is arranged within the channel, where air flowing from the openings flows through the exchanger so that the air is heated. An axial ventilator (40) conveys the air from the openings, and is arranged behind the exchanger in the flow direction. Another air inlet opening is arranged between the exchanger and the ventilator in the direction, and actuated by a one-way valve, which opens during falling below a preset low pressure in a region of the latter inlet opening.
