The Airlet 600 is a unique make-up air inlet that automatically adjusts the incoming airflow according to interior humidity levels of the room in which the unit is installed. The rectangular grill in the picture is what you will see mounted on your wall. . The tube is 4″ in diameter, and almost 14″ in length. It can be cut easily to fit thinner walls. The vent opens when indoor humidity is 50% and closes when it goes down to 35%.