Whole-house fan  A whole house fan is a type of fan, commonly venting into a building's attic, designed to circulate air in a home or building. It is sometimes confused with a powered attic ventilator, which exhausts hot air from the attic to the outside through an opening in the roof or gable at a low velocity. A whole house fan pulls air out of a building and forces it into the attic space or, in the case of homes without attics, through an opening in the roof or an outside wall. This causes a positive pressure differential in the attic forcing air out through the gable and/or soffit vents, while at the same time producing a negative pressure differential inside the living areas which draws air in through open windows. Powered attic ventilators, by comparison, only serve to remove some hot air from the attic. Intake air comes directly from outside, instead of from the house interior. Attic ventilators are used when air intake from the house is not desirable, such as when the interior is air-conditioned. Typical diameters are , , and , having motors of , and using approximately 190 to 560 watts of electric power at high speed. Before the development of electrical power the principle of cooling a building by designing it to draw in cooler air from below and vent it from the top was known. In the absence of the forced air movement produced by a fan, careful design to promote cooling air flow was required. Thomas Jefferson, US president from 1801 to 1809, was personally involved in the designs of his residences at Monticello and Poplar Forest, and was aware of these techniques. Monticello had a large central hall and aligned windows designed to allow a cooling air-current to pass through the house, with an octagonal cupola at the top of the house drawing hot air up and out through natural convection. Whole house fans were the only method for cooling homes in the early 1900s. Air conditioning was invented by Carrier in 1907 but did not become popular until the 1950s. Whole house fans are still ideal for cooling homes when the air outside is cooler than the air inside. There are four types of whole house fans: