A window fan is a fan

Fan (mechanical)

A mechanical fan is a machine used to create flow within a fluid, typically a gas such as air.A fan consists of a rotating arrangement of vanes or blades which act on the air. Usually, it is contained within some form of housing or case. This may direct the airflow or increase safety by preventing...

 designed to be placed inside the frame of a window

Window

A window is a transparent or translucent opening in a wall or door that allows the passage of light and, if not closed or sealed, air and sound. Windows are usually glazed or covered in some other transparent or translucent material like float glass. Windows are held in place by frames, which...

. Window fans have been used for many decades to maintain comfortable temperatures

Room temperature

-Comfort levels:The American Society of Heating, Refrigerating and Air-Conditioning Engineers has listings for suggested temperatures and air flow rates in different types of buildings and different environmental circumstances. For example, a single office in a building has an occupancy ratio per...

 and ventilation

Ventilation (architecture)

Ventilating is the process of "changing" or replacing air in any space to provide high indoor air quality...

 within one's home, and operate at a tiny fraction of the cost of central air conditioning.

Design and operation

Window fans are designed to fit into open windows, and often have expandable side panels to cover the entire window opening. Most units have either one large-sized fan, or two small-sized fans mounted side by side.

Most window fans have 2 or 3 speeds, and some are electrically reversible so that the user can switch between intake and exhaust modes.

Manufacturers have added features to their models over the years. Some fans contain thermostat

Thermostat

A thermostat is the component of a control system which regulates the temperature of a system so that the system's temperature is maintained near a desired setpoint temperature. The thermostat does this by switching heating or cooling devices on or off, or regulating the flow of a heat transfer...

s which will turn the fan off if the temperature drops to a certain point, or adjustable grille

Grille

A grille or grill is an opening of several slits side by side in a wall or metal sheet or other barrier, usually to let air or water enter and/or leave but keep larger objects including people and animals in or out.-Spelling:In the United States, "grille" is used to differentiate the automotive...

s to direct the flow of air in a particular direction. Other fans offer sophisticated electronic features such as a remote control

Remote control

A remote control is a component of an electronics device, most commonly a television set, used for operating the television device wirelessly from a short line-of-sight distance.The remote control is usually contracted to remote...

, automatic shut off timer

Timer

A timer is a specialized type of clock. A timer can be used to control the sequence of an event or process. Whereas a stopwatch counts upwards from zero for measuring elapsed time, a timer counts down from a specified time interval, like an hourglass.Timers can be mechanical, electromechanical,...

s, or dimmer-like variable speed motor controls.

Most window fans today are made of plastic. Many older fans, however, are made of metal and are more solid. The more powerful window fans can be used to circulate air throughout a house, acting as a whole house fan

Whole house fan

A whole-house fan is a type of fan, or exhaust system commonly venting into a building's attic, designed to pull hot air out of the building. It is sometimes confused with an attic fan....

.