An air door or air curtain is a machine used for separating two spaces from each other. The machine is mainly used at doors into buildings. The most common setup for air curtains is a downward-facing blower fan placed over an opening. Normally this opening is a door to a building. The fan stops outside air from entering the building to keep the building warm or cool. The air curtain also keeps insects out by making turbulence. The fan must be powerful enough to generate a jet of air that can reach the floor. They can also be heated or cooled as well to provide space heating or space cooling to an area.