Ceiling Fan

A ceiling fan is a mechanical fan mounted on the ceiling of a room or space, usually electrically powered, that uses hub-mounted rotating blades to circulate air. They cool people effectively by increasing air speed. Fans do not reduce air temperature or relative humidity, unlike air-conditioning equipment but create a cooling effect by helping to evaporate sweat and increase heat exchange via convection.

History
The first rotary ceiling fans appeared in the early 1860s and 1870s in the United States. At that time, they were not powered by any form of electric motor. Instead, a stream of running water was used, in conjunction with a turbine, to drive a system of belts which would turn the blades of two-blade fan units. These systems could accommodate several fan units, and so became popular in stores, restaurants, and offices. Some of these systems survive today, and can be seen in parts of the southern United States where they originally proved useful.

Uses
Ceiling fans have multiple functions. Fans increase mixing in a ventilated space, which leads to more homogenous environmental conditions. Moving air is generally preferred over stagnant air, especially in warm or neutral environments, so fans are useful in increasing occupant satisfaction. Because fans do not change air temperature and humidity, but move it around, fans can aid in both the heating and cooling of a space. Because of this, ceiling fans are often an instrumental element of low energy HVAC, passive cooling or natural ventilation systems in buildings.

Types
Ceiling fans can be classified into three main categories based on their use and functionality. Each type offers some unique advantages over the others and hence is suitable for a specific application. These include household, industrial and large-diameter fans. Many styles of ceiling fans have been developed over the years in response to several different factors such as growing energy-consumption consciousness and changes in decorating styles. The advent and evolution of new technologies have also played a major role in ceiling fan development.

Installation
A typical ceiling fan weighs between 8 and 50 pounds when fully assembled. While many junction boxes can support that weight while the fan is hanging still, a fan in operation exerts many additional stresses—notably torsion—on the object from which it is hung; this can cause an improper junction box to fail. For this reason, in the United States the National Electric Code states that ceiling fans must be supported by an electrical junction box listed for that use. It is a common mistake for homeowners to replace a light fixture with a ceiling fan without upgrading to a proper junction box.