Grille with gravity shutters is designed to allow an air by ventilation system flow only in one way. When the fan put an air out of the building – feathers rise to make an air flow and when a fan didnt work – a feathers falls and close a flow between inside and outside to resist getting cold air, dirt and insects to the room. This grille model comes in 4 circular collar sizes, allowing for convenient installation of ventilation systems in the most common sizes.