Big Ass Fans saves you money in the winter by circulating the hot air trapped at the ceiling back down to the floor. We do this by slowing the fan’s rotation, not reversing it, below the level of a perceived breeze. We asked our resident air movement guru, Dr. Richard Aynsley, to give us a few words on why it’s actually a bad idea to reverse ceiling fans in the winter. Below is what he gave us:
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.
The use of ceiling fans for air movement to provide 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 4.7°F while maintaining the same thermal comfort. Raising the thermostat 1°F typically saves between 3% and 4% of cooling energy cost*. Raising the thermostat 5°F 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 10°F 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.
Move that hot air trapped at the ceiling on down to the floor.
Circulation of indoor air at 3 to 4 times per hour, particularly in spaces with high ceilings, can allow the thermostat to be set 10°F to 15°F lower while maintaining the same thermal comfort. For each 1°F 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 5°F from 85°F to 80°F will typically save approximately 15% of heating energy cost per day.** Field data from clients using Big Ass ceiling fans indicates monthly winter gas consumption can be cut by up to 30%.
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.
*Exeloncorp, formerly Consolidated Edison, www.exeloncorp.com
** US Dept of Energy, www.eere.energy.gov
Richard Aynsley, Ph.D., M.ASHRAE,
B.Arch (Hons I), MS(Arch.Eng), Registered Architect, QLD.
Director Research & Development
Big Ass Fan Company
Lexington, KY, USA
