A VF-BDT Tubular Ventilating 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 introducing slow movement into the otherwise still, hot air of a room. Fans do not reduce air temperature, unlike air-conditioning equipment but create a wind chill effect by evaporating sweat in the summer. In fact, they heat up the air slightly due to the waste heat from the motor and friction between the moving air. Fans use significantly less power than air conditioning as 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.