Ceiling fan
A ceiling fan is a 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. Fans may add a small amount of heat to the room due to friction and waste heat from the motor. Fans use significantly less power than air conditioning as cooling air is thermodynamically expensive . In the winter a ceiling fan can also be used to bring warm air, which naturally rises, back down to occupants. This can affect both thermostat readings and occupants' comfort, thereby improving climate control energy efficiency. History. Punkah style ceiling fans are based on the earliest form of a fan, which was first invented in India around 500 BC. These were cut from an Indian palmyra leaf which forms its rather large blade, moving slowly in a pendular manner. Originally operated manually by a cord and nowadays powered electrically using a belt-driven system, these punkahs move air by going  to and fro. In comparison to a rotating fan, it creates a gentle breeze rather than an airflow. 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. The electrically powered ceiling fan was invented in 1882 by Philip Diehl. He had engineered the electric motor used in the first electrically powered Singer sewing machines, and in 1882 he adapted that motor for use in a ceiling-mounted fan. Each fan had its own self-contained motor unit, with no need for belt drive. Almost immediately he faced fierce competition due to the commercial success of the ceiling fan. He continued to make improvements to his invention and created a light kit fitted to  the ceiling fan to combine both functions in one unit. By World War I most ceiling fans were made with four blades instead of the original two, which made fans quieter and allowed them to circulate more air. The early turn-of-the-century companies who successfully commercialized the sale of ceiling fans in the United States were what is today known as the Hunter Fan Company, Robbins & Myers, Century Electric, Westinghouse Corporation and Emerson Electric. By the 1920s, ceiling fans became commonplace in the United States and had started to take hold internationally. From the Great Depression of the  1930s, until the introduction of electric air conditioning in the 1950s, ceiling fans slowly faded out of vogue in the U.S., almost falling into total disuse in the U.S. by the 1960s; those that remained were considered items of nostalgia. Meanwhile, energy efficient ceiling fan became very popular in other countries, particularly those with hot climates, such as India and the Middle East, where a lack of infrastructure and/or financial resources made energy-hungry and complex freon-based air conditioning equipment impractical. In 1973, Texas entrepreneur H. W. (Hub) Markwardt began importing ceiling fans into the United States that were  manufactured in India by Crompton Greaves, Ltd. Crompton Greaves had been manufacturing ceiling fans since 1937 through a joint venture formed by Greaves Cotton of India and Crompton Parkinson of England. These Indian manufactured ceiling fans caught on slowly at first, but Markwardt's Encon Industries branded ceiling fans (which stood for ENergy CONservation) eventually found great success during the energy crisis of the late 1970s and early 1980s since they consumed less energy than the antiquated shaded pole motors used in most other American made fans. The fans became the energy-saving appliances for residential and commercial use by  supplementing expensive air conditioning units with a column of gentle airflow. Due to this renewed commercial success using ceiling fans effectively as an energy conservation application, many American manufacturers also started to produce, or significantly increase the production of, ceiling fans. In addition to the imported Encon ceiling fans, the Casablanca Fan Company was founded in 1974. Other American manufacturers of the time included the Hunter Fan Co. (which was then a division of Robbins & Myers, Inc), FASCO (F. A. Smith Co.), and Emerson Electric; which was often branded as Sears-Roebuck. Through the 1980s and 1990s, ceiling fans remained  popular in the United States. Many small American importers, most of them rather short-lived, started importing ceiling fans. Throughout the 1980s, the balance of sales between American-made ceiling fans and those imported from manufacturers in India, Taiwan, Hong Kong and eventually China changed dramatically with imported fans taking the lion's share of the market by the late 1980s. Even the most basic U.S-made fans sold for $200 to $500, while the most expensive imported fans rarely exceeded $150. Since 1980, ceiling fan technology has not evolved much until recently, with the availability of  energy-efficient, remote/app controlled brushless DC fans to the masses. However, important inroads have been made in design by companies such as Monte Carlo, Minka Aire, Quorum, Craftmade, Litex and Fanimation - offering higher price ceiling fans with more decorative value. In 2001, Washington Post writer Patricia Dane Rogers wrote, "Like so many other mundane household objects, these old standbys are going high-style and high-tech." 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. Depending on the energy use of the fan system, fans can be an efficient way to improve thermal comfort by allowing for a higher ambient air temperature while keeping occupants comfortable. Fans are an especially economic choice in warm, humid environments. Ceiling fans can  be controlled together in a shared space, and can also be individually controlled in a home or office setting.  In an office environment, individually controlled ceiling fans can have a significant positive impact on thermal comfort, which has been shown to increase productivity and satisfaction among occupants. Ceiling fans aid in the distribution of fresh air in both mechanically ventilated and naturally ventilated spaces. In naturally ventilated spaces, ceiling fans are effective at drawing in and circulating fresh outdoor air.  In mechanically ventilated spaces, fans can be focused to channel and circulate conditioned air in a room. Direction. The direction  that a fan spins should change based on whether the room needs to be heated or cooled. Unlike air conditioners, fans only move air—they do not directly change its temperature. Therefore, ceiling fans that have a mechanism for reversing the direction in which the blades push air (most commonly an electrical switch on the unit's switch housing, motor housing, or lower canopy) can help in both heating and cooling. While ceiling fan manufacturers (mainly Emerson) have had electrically reversible motors in production since the 1930s, most fans made before the mid-1970s are either not reversible at all or  mechanically reversible (have adjustable blade pitch) instead of an electrically reversible motor. In this case, the blades should be pitched with the upturned edge leading for downdraft, and with the downturned edge leading for updraft. Hunter's "Adaptair" mechanism is perhaps the most well-known example of mechanical reversibility. For cooling, the fan's direction of rotation should usually be set so that air is blown downward (Usually counter-clockwise from beneath, but dependent upon manufacturer). The blades should lead with the upturned edge as they spin. The breeze created by a ceiling fan creates a wind chill effect, speeding  the evaporation of perspiration on human skin, which makes the body's natural cooling mechanism much more efficient. As a result of this phenomenon, the air conditioning thermostat can be set a few degrees higher than normal when a fan is in operation, greatly reducing power consumption. Since the fan works directly on the body, rather than by changing the temperature of the air, it is recommended to switch all ceiling fans off when a room is unoccupied, to further reduce power consumption. In some cases, like when fan is near walls like in a hallway, updraft may cause better  airflow. Also another example how updraft can cause better cooling is that fan is in middle of a bedroom with a loft bed near a wall, meaning breeze can be felt better when airflow is coming from the top. For heating, ceiling fans should be set to blow the air upward. Air naturally stratifies, i.e. warmer air rises to the ceiling while cooler air sinks, meaning that colder air settles near the floor where people spend most of their time. A ceiling fan, with its direction of rotation set so that air is drawn upward, pulls the colder air off  the floor, forcing the warmer air at ceiling level to move down to take its place, without blowing a stream of air directly at the occupants of the room. This action works to equalize, or even out the temperature in the room, making it cooler at ceiling level, but warmer near the floor. Thus the heating thermostat in the area can be set a few degrees lower to save energy while maintaining the same level of comfort. Though reversible models of industrial-grade ceiling fans do exist, most are not reversible. High ceiling heights in most industrial applications render reversibility  unnecessary. Instead, industrial ceiling fans typically de-stratify heat by blowing hot air at ceiling level directly down toward the floor. Blade shape. Residential ceiling fans, which are almost always reversible, typically use flat, paddle-like blades, which are equally effective in downdraft and updraft. Industrial ceiling fans typically are not reversible and operate only in downdraft, and therefore are able to make effective use of blades that are contoured to have a downdraft bias. More recently, however, residential ceiling fan designers have been making increasing use of contoured blades in an effort to boost ceiling fan efficiency. This contour , while serving to effectively boost the fan's performance while operating in downdraft, can hinder performance when operating in updraft. Air conditioning. The most commonplace use of ceiling fans today is in conjunction with an air conditioning unit. Without an operating ceiling fan, air conditioning units typically have both the tasks of cooling the air inside the room and circulating it. Provided the ceiling fan is properly sized for the room in which it is operating, its efficiency of moving air far exceeds that of an air conditioning unit, therefore, for peak efficiency, the air conditioner should be set to  a low fan setting and the ceiling fan should be used to circulate the air. Flicker and strobing. Ceiling fans can cause shadow flicker and strobing when they are installed close to ceiling lights. This is due to the fan blades intermittently blocking the light, causing shadows to appear across the room's interior surface leading to visual discomfort. Flicker can be avoided by using drop-down light fixtures, which are mounted at the same height, or lower, than the ceiling fan. Parts. The key components of a ceiling fan are the following: Other components, which vary by model and  style, can include: Operation. The way in which a fan is operated depends on its manufacturer, style, and the era in which it was made. Operating methods include: Classifications. 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. Types. 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. Following is a list of major ceiling fan styles and their defining characteristics: Safety concerns with 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 (document NFPA 70 , Article 314) 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. Low-hanging fans/danger to limbs. Another concern with installing a ceiling fan relates to the height of the blades relative to the floor. Building codes throughout the United States prohibit residential ceiling fans from being mounted with the blades closer than seven feet from the floor; this sometimes proves, however, to not be high enough. If a ceiling  fan is turned on and a person fully extends his or her arms into the air, as sometimes happens during normal tasks such as dressing, stretching or changing bedsheets, it is possible for the blades to strike their hands, potentially causing injury. Also, if one is carrying a long and awkward object, one end may inadvertently enter the path of rotation of a ceiling fan's blades, which can cause damage to the fan. Building codes throughout the United States also prohibit industrial ceiling fans from being mounted with the blades closer than 10 feet from the floor for these  reasons. Section::::"MythBusters": "Killer Ceiling Fan". In 2004, "MythBusters" tested the idea that a ceiling fan is capable of decapitation if an individual was to stick his or her neck into a running fan. Two versions of the myth were tested, with the first being the "jumping kid", involving a kid jumping up and down on a bed, jumping too high and entering the fan from below and the second being the "lover's leap", involving a husband leaping towards his bed and entering the fan side-on. Kari Byron and Scottie Chapman purchased a regular household fan and also an  industrial fan, which has metal blades as opposed to wood and a more powerful motor. They busted the myth in both scenarios with both household and industrial fans, as tests proved that residential ceiling fans are, apparently by design, largely incapable of causing more than a minor injury, having low-torque motors that stop quickly when blocked and blades composed of light materials that tend to break easily if impacted at speed (the household fan test of the "lover's leap" scenario actually broke the fan blades.) They did find that industrial fans, with their steel blades and higher speeds , proved capable of causing injury and laceration - building codes require industrial fans to be mounted with blades 10 feet above the floor, and the industrial fan test of the "lover's leap" scenario produced a lethal injury where the fan sliced through the jugular and into the vertebrae - but still lost energy rapidly once blocked and were unable to decapitate the test dummy. Wobble. Wobbling is usually caused by the weight of fan blades being out of balance with each other. This can happen due to a variety of factors, including blades being warped, blade irons being bent, blades or  blade irons not being screwed on straight, or weight variation between blades. Also, if all the blades do not exert an equal force on the air (because they have different angles, for instance), the vertical reaction forces can cause wobbling. Wobble can also be caused by a motor flaw, but that very rarely occurs. Wobbling is not affected by the way in which the fan is mounted or the mounting surface. Contrary to popular misconception, wobbling alone will not cause a ceiling fan to fall. Ceiling fans are secured by clevis pins locked with either split pins or R-clips , so wobbling will not have an effect on the fan's security, unless of course, the pins/clips were not secured. To date, there are no reports of a fan wobbling itself off the ceiling and falling. However, a severe wobble can cause light fixture shades or covers to gradually loosen over time and potentially fall, posing a risk of injury to anyone under the fan, and also from any resulting broken glass. When the MythBusters were designing a fan with the goal of chopping off someone's head, Scottie used an edge finder to find the exact center of  their blades with the aim of eliminating potentially very dangerous wobbling of their steel blades. Wobbling may be reduced by measuring the tip of each blade from a fixed point on the ceiling (or floor) and ensuring each is equal. If the fan has a metal plate between the motor and blade, this may be gently adjusted by bending. It can also be reduced by making sure all blades have the same pitch, and all blades have the same distance from adjacent blades. It can also be reduced by having balancing weight on the blades. Disposability. Ceiling fan motors often  have chemicals inside to prevent them from burning out, rendering them not disposable. When placed in a garbage truck, chemicals inside the motor can cause pollution which is bad for human health, or in some very rare cases, cause an explosion. The same can happen if ceiling fans are left inside a building when the building is demolished. Ceiling fans should be recycled, sold or donated.