A fan attached to a room’s ceiling is known as a ceiling fan. Like other fans, it is used to provide comfort for building occupants by circulating air within a room. 
 Fun Facts About Ceiling Fans

• An adult human cannot be decapitated by a ceiling fan, according to the TV show "MythBusters."  A powerful, industrial-strength fan might be able to damage a skull or slice a person’s neck, however. 
• Ceiling fans were first used in the United States in the 1860s. They were powered by a system of belts driven by a stream of running water. 
• Unlike air conditioners, fans do not actually cool the air, which is why they merely waste electricity when they circulate air in an unoccupied room.

Ceiling Fan Components
A ceiling fan is comprised of the following parts:

• electric motor:  varies with the size of the fan and its application; 
• blades:  typically, two to six spinning, precision-weighted blades made from metal, wood or plastic; industrial fans typically have three blades, while residential models have four or five; 
• blade irons:  connect the blades to the motor; 
• safety cable: on heavy fans, these are required to hold the fan in place in case the support housing fails; 
• flywheel:  connects the blade irons to the motor; 
• ceiling mount:  designs include ball-in-socket and J-hook; 
• downrod:  used where ceiling fans are suspended from high ceilings; 
• motor housing:  protects the fan motor from dust and its surroundings; may also be decorative; and 
• lamps: may be installed above, below or inside the motor housing.

Common Fan Defects

• The fan falls. A ceiling fan that breaks free from its ceiling mount can be deadly. Fans must be supported by an electrical junction box listed for that use, according to the National Electric Code, and a fan brace box will need to be installed. While a particular junction box might support a fully assembled fan, during operation, it will exert additional forces (notably, torsion) that can cause the support to fail. Homeowners often overlook this distinction by carelessly replacing light fixtures with ceiling fans without upgrading the junction box, which should clearly state whether it’s rated to hold a ceiling fan. 
• The fan wobbles. This is a common and distracting defect that is usually caused when fan blades are misaligned from one another. Specific problems stem from minute differences in the size or weight of individual blades, warping, bent blade irons, or blades or blade irons that are not screwed in tightly enough. The ceiling mount may also be loose. Wobbling is not caused by the ceiling or the particular way that the fan was mounted. Wobbling will not cause the fan to fall, and there have been no such reports. Wobbling can, however, cause light fixture covers or shades to loosen and potentially fall. These items should be securely attached, with all screws tightly set in place. An easy way to tell if the blades are not on the same plane is to hold a yardstick or ruler against the ceiling and measure the distance that the tip of each blade is from the ceiling by manually pushing the blades. A homeowner can carefully bend the misaligned blade back into place. Blades can also be corrected in this way if measurement reveals that they are not equidistant from one another.  
• There is inadequate floor-to-ceiling blade clearance. No part of the fan blades of a residential ceiling fan (usually having four or more blades) should be closer than 7 feet from the floor in order to prevent inadvertent contact with the blades. Downward air movement is maximized when the fan blades are around 8 or 9 feet from the floor. For high ceilings, the fan may be hung to a desired height. Low-profile fan models are available for ceilings that are lower than 8 feet from the floor. Also, fan blades should be at least 18 inches from walls. For commercial ceiling fans (usually having three blades), no part of the fan blades should be closer than 10 feet from the floor in order to prevent inadvertent contact with the blades.  Underwriters Laboratories UL 507 Section 70.2.1 says: 
  
  "The blades of a ceiling-suspended fan shall be located at least 3.05 m (10 feet) above the floor when the fan is installed as intended."

Underwriters Laboratories makes exceptions if the fan blade edges are thick and the fan is turning slowly.

• Blades are turning in the wrong direction. In the winter months, the leading edge of the fan's blades should be lower than the trailing edge in order to produce a gentle updraft, which forces warm air near the ceiling down into the occupied space below. In the summer, the leading edge of the fan's blades should be higher as the fan spins counter-clockwise to cool occupants with a wind-chill effect. On most models, the fan direction can be reversed with an electric switch located on the outside of the metal housing, but the same effect can be achieved on other models by unscrewing and remounting the fan blades. 
• An indoor fan is not designed for exterior use. Ordinary indoor ceiling fans are unsafe to use outdoors or in humid environments, such as bathrooms.  They will wear out quickly. Fans that are rated “damp” are safe for humid environments, but they, too, should never be used where they might come into contact with liquid water. Only fans that are rated “wet” are safe for such use, as they incorporate features such as all-weather, UV-resistant blades, sealed motors, rust-resistant housing, and stainless steel hardware.

In summary, properly installed and maintained ceiling fans can inexpensively cool or warm building occupants.