Abstract: A ventilation device for ventilating rooms in dwelling locations, or other spaces such as workman's barracks, cold storage and freezing rooms, and the like. An elongated housing is provided with a fan at one end with a reversible motor for controlling the direction of the fan. Heat holdover means comprising a series of heat-exchange plates is positioned in the housing and parallel to the direction of the air flow. Thermostats are provide at both ends of the housing for sensing the temperature of the air flowing by and thereby operating the reversible motor, thus controlling the direction of the fan.