Window Fans
As the name implies, window fans are located in window openings. Depending on how they are set, they either pull warm inside air out of the home or blow outside air in. Most people do not like the blast of outside air window fans create when they pull air into the home. So more often, they are used to ventilate the home by moving inside air out so it can be replaced with cooler “outside” air.
Designed to be placed in windows, these fans do a great job of pulling fresh outside air into the home. Some models are reversible, so you can also exhaust or pull air out of the home.
To be most effective, window fans should be located in a window on the hottest or the downwind side of the house so natural air currents will assist rather than fight them. Also, because hot air naturally rises, it’s a good idea to put them in windows toward the top of the home, (i.e. an upstairs window of a two-story home). Then, open one or more downstairs windows on the coolest or upwind side of the house. Interior doors between the two windows must remain open to allow air flow.
Window fans are designed to fit medium or large sized windows and have expandable side barriers to cover the entire window opening. Most units have either one large sized bladed fan, or two small sized bladed fan units mounted side by side. Rarely, some window fans have used a blower wheel, which is similar in appearance to a hamster wheel.
Most window fans have 3 speed electric motors. Some allow the motor direction to be reversed electrically via a switch, allowing either fresh air to be drawn in from the outside, or inside air to be blown outside. Fans with 2 motors typically have separate controls for each motor.
Manfacturers have added features to their models over the years. Some fans contain thermostats which will turn the fan off if the temperature drops to a certain point, or adjustable grilles to direct the flow of air in a particular direction. Other fans offer sophisticated electronic features such as remote control, automatic shut off timers, or variable speed motor controls.
Window fans work well in houses that are fairly “open” inside, but do not do well in homes with long, narrow hallways or many interior partitions that may interfere with air “flow.”
Source:Home Energy Library and en.wikipedia.org