The Holmes HAWF2043 double window fan exhausts stale hot air from inside or draws in fresh cool air from the outside. It can also do both simultaneously as well; each of the two fans can be set independently to exhaust out or draw in. Operable either manually or automatically with its thermostat turning the fans on and off to maintain a selected temperature this fan has a one-touch electronic control and two speeds so it may be adjusted to your specified conditions. It s designed to fit double-hung vertical-slider and casement windows. A built-in 7-inch extender and extra 3-inch extender permit custom fitting for windows 24-37 wide. The fans have six blades and measure 8-1/2 inches in diameter. The unit is 12-1/4 inches high arrives fully assembled.