As an architect and engineer, I can understand why the architects for some projects seek truly reversible air movement from ceiling fans. This simplistic notion of ceiling fan use has been around for decades. Let me explain as an architect with a Masters degree in Engineering, why this is not a sound idea.
Use of ceiling fans for air movement to energy-efficient summer comfort is straight forward. In summer, providing air movement of 160 fpm in the occupied zone will allow the thermostat to be raised 4.7F while maintaining the same thermal comfort. Raising the thermostat 1F typically saves between 3% and 4% of cooling energy cost*. Raising the thermostat 5F will typically save between 15% and 20% of cooling energy costs. In summer, providing air movement of 230 fpm (2.6 mph) in the occupied zone of an air conditioned space will allow the thermostat to be raised 10F while maintaining the same thermal comfort. This thermostat increase would save 30% to 40% of air conditioning cooling costs.
Use of ceiling fans at low speed with air flow downward for energy efficiency by destratification in winter is well established. Circulation of indoor air at 3 to 4 times per hour, particularly in spaces with high ceilings, can allow the thermostat to be set 10F to 15F lower while maintaining the same thermal comfort. For each 1F the thermostat is lowered in winter the typical heating energy cost saving is 1% per 8 hour period, or 3% per day. Lowering the thermostat in winter by 5F from 85F to 80F will typically save approximately 15% of heating energy cost per day.
Here is the catch. If the airflow from the ceiling fan is reversed with airflow upward, even at low speed, the velocity of air across the ceiling above the fan is high, typically around 400 fpm. At this speed, the heat transfer coefficient at the ceiling is around 1.87 Btu/h.ft2.F. However if the ceiling fan is running at low speed with airflow downward the airflow across the ceiling is low, typically 80 fpm. At this speed, the heat transfer coefficient at the ceiling is around 0.051 Btu/h.ft2.F.
In short, reversing the air flow direction from ceiling fans in winter the heat loss through the ceiling is increased by a factor of around 3.7 times due the increase in surface conductance.
Big Ass Fans is the leading manufacturer of huge ceiling fans for large industrial, agricultural and commercial settings. Big Ass Fans range in size from 6 feet to 24 feet in diameter, using low horsepower motors to generate energy-efficient air movement for large spaces. Big Ass Fans can be used year round to provide occupant comfort and energy savings. There are over 35,000 Big Ass Fans installed worldwide in applications such as manufacturing plants, distribution centers, horse arenas, dairy barns, stadiums, art galleries, restaurants, health clubs and even zoos! Product solutions include Powerfoil, PowerfoilPlus, Pivot, the 8-ft. Mobile Vertical, and the new Element designed for commercial spaces. Big Ass Fans have been featured in Inc. Magazine, Entrepreneur, Fast Company, and television programs such as Planet Green, Discovery Channel Canada and the National Geographic Channel. For more information on Big Ass Fans, visit the website at http://www.bigassfans.com
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