Not to be confused with a whole-house fan
An attic fan is a ventilation fan which regulates the heat level of a building's attic by exhausting hot air. A thermostat is used to automatically turn the fan off and on, while sometimes a manual switch is used. An attic fan can be gable mounted or roof mounted. Additional vents are required to draw in fresh air as the hot air is exhausted. Attic fans are typically used in warmer months, when temperatures in a building's attic may very well exceed 120 degrees Fahrenheit (48.9 degrees Celsius).
Attic fans may be wired several different ways. Usually the fan is hardwired directly into an electric circuit, and is controlled by a thermostat most commonly ranging from 60 to 120 degrees Fahrenheit (15.6 to 48.9 degrees Celsius). It may also be hardwired but controlled by a switch. Another popular method is to have the fan plugged into an electric socket mounted nearby.
Because an attic fan is made of metal, and is also subject to lightning strikes if mounted on a roof, grounding is extremely important. On some attic fans, grounding is achieved through a screw mounted on the thermostat box of the attic fan. The grounding wire is attached to the box via the screw and thereby grounds the external metal components. Most modern attic fans have this as well as a grounding wire connected directly to the motor, thereby ensuring all parts are grounded.
Though every situation is different, there is some controversy about the effectiveness of attic fans. It has been theorized that in some cases the negative pressure that an attic fan produces in the attic can cause conditioned air from living spaces to be pulled into the attic through cracks and lighting fixtures. This loss of conditioned air can negate the energy conservation gains that you might expect to realize by forced cooling of the attic with a fan. There is an extensive discussion thread on the website Hvac-Talk.com here:  Also, in the possible event of a fire, an operating attic fan could, in theory, spread the flames because of the air movement. Some recently marketed attic fans will shut off if temperature in the space rises above 180 degrees Fahrenheit (82 degrees Celsius). The effectiveness of the attic fan generally depends on the size and insulation of the space, as well as operating expenses.
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