Ceiling fan
A ceiling fan is a mechanical fan mounted on the ceiling of a room or space, usually electrically powered, suspended from the ceiling of a room, that uses hub-mounted rotating blades to circulate air. Ceiling fans typically rotate more slowly than other types of circulating fans, such as electric desk fans. They cool people effectively by introducing slow movement into the otherwise still, hot air of a room. Fans never actually cool air, unlike air-conditioning equipment, they in fact heat the air due to the waste heat from the motor and friction, but use significantly less power (cooling air is thermodynamically expensive). Conversely, a ceiling fan can also be used to reduce the stratification of warm air in a room by forcing it down to affect both occupants' sensations and thermostat readings, thereby improving climate control energy efficiency.
Thermal destratification
Because of their size, HVLS fans are normally installed in new construction, rather than retrofits, as the roof structure may have to be redesigned to accommodate the increased weight and size. It's not uncommon to require the relocation of lights, due to strobing as large fan blades pass under them, and sprinkler systems, which typically require unobstructed access to the floor to meet fire code. When used in the summer to encourage evaporative cooling, HVLS fans are run forward, blowing air at the floor. When used for destratification in the winter, the fans are run in reverse, blowing air towards ceiling which then circulates around the room. The height at which HVLS fans can be effective is limited compared to axial destratification fans.