RotoEffect - revolving door air curtain A revolving door will act as a low-speed fan, forcing a mass of cold air into a room with each rotation. By combining an air curtain with a revolving door system this cold air is prevented from entering. Three types of model are available. A Door-Internal unit that is mounted above the door and blows warm air into its inner sections, a Room-Side unit that is also mounted above the door but blows across the inner opening, and a Room-Side External unit that blows across the inner opening but is mounted on the inside of the building.