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Destratifying Heat With Large-Diameter, Low-Speed Fans

May 10, 2011
A simplified method of equalizing air temperatures from floor to ceiling

Ceiling height, insulation values, the presence or absence of floor-to-ceiling windows—all affect the regulation of indoor temperatures. With the natural tendency of heat to rise, a heating system has to work overtime to maintain the temperature set point at the occupant/thermostat level. Blending stratified warm and cool air through the use of large-diameter, low-speed fans eliminates the draft associated with typical fan circulation to achieve a comfortably heated space.

Hot Meets Cold
Stratification occurs because hot air is less dense than cold air. The air coming out of a heating system or produced by manufacturing equipment is approximately 5- to 7-percent lighter than the ambient air and collects at the ceiling. This results in a vertical temperature difference of 5°F to 30°F.

Figures 1 and 2 show the difference in vertical temperature in a semi-insulated hangar in Frankfort, Ky., with large-diameter, low-speed fans off for a week and on for a week, respectively. With the fans off, the average temperature difference between the floor and ceiling is more than 5°F. With the fans on, floor-to-ceiling temperatures are nearly uniform.

'Angle of Attack'
Effective air movement does not occur through fan rotation alone. The number, the pitch, and the type of airfoils, or blades, also play important roles.

A steep "angle of attack" increases drag and, thus, requires a larger motor. Additionally, it moves less air, which results in greater operating costs and lower aerodynamic efficiency. A nearly horizontal, or flat, airfoil, meanwhile, typically moves little air.

The most efficient air movement is accomplished with a fan with 10 moderately pitched, narrow, aerodynamic airfoils. Ten airfoils provides a more uniform (less choppy or pulsating) airflow pattern that allows a fan to spin at an increased number of revolutions per minute without causing a draft. This reduces the number of fans required to destratify a space.

The Method of the Means
An estimate of winter energy savings is the reduction in heat loss through a building’s envelope caused by the reduced differential between outdoor-air temperatures and floor- and ceiling-level indoor-air temperatures. Building occupants can access through their local utility company data regarding the amount of gas or other fuel used during heating season, while local climate records can provide average outdoor-air temperature during a particular heating season. This data can be used to calculate the overall rate of heat loss through a building’s envelope in British thermal units per square foot per degree-day.

Measured Savings
Considering one of the biggest cost factors with any structure is energy consumption, finding solutions to curb usage while maintaining necessary comfort is economically and environmentally sound.

In the distribution warehouses of Federated Co-operative Ltd., a retail cooperative with members throughout western Canada, a fair amount of heat was believed to be wasted at the ceiling. The company's goal was to decrease the rate at which the buildings shed heat through the roof using destratification.

As a pilot test, Trevor Carlson, Federated's environmental and technical-services manager, installed 24-ft-diameter, low-speed fans in the 80,000-sq-ft loading-dock section of Federated's 300,000-sq-ft warehouse in Saskatoon, Saskatchewan.

"We looked at what our degree data was like and calculated our heating index for the year prior and the year after installing the fans," Carlson said. "After a very short period of operation, we noticed multiple benefits. The workers in the warehouse actually wanted the temperature decreased in the winter because they were too warm. That was very encouraging for us, as we were able to change the set points on the thermostats because the fans were bringing the heat down, keeping workers comfortable while reducing our costs."

The year before the fans were installed, the heating index was 4.49 Btu per square foot per degree-day. The year the fans were installed, the heating index was 3.99 Btu per square foot per degree-day. The year after the fans were installed, the heating index was 3.61 Btu per square foot per degree-day.

"We noticed a pretty significant decrease, and it works out to a roughly 10-percent reduction in natural-gas consumption," Carlson said. "It was a 10-percent reduction when we saw a 20-percent increase in natural-gas rates. We believe we saved $18,587 the first year in natural-gas consumption as a result of the fans."

At Fred and Sara Machetanz Elementary School in Wasilla, Alaska, a 12-ft-diameter, low-speed fan helps destratify the air in a 1,768-sq-ft atrium. Installed at a height of 36 ft, the fan is incorporated into the building-automation system to help regulate temperatures between the daytime and nighttime thermostat set points.

During winter—most notably, October to February—the daytime thermostat set point of 68°F is reduced to 55°F overnight.

"The system kicks on at 5:30 in the morning to make sure the building is comfortable by the time staff and faculty start arriving at 6:30 a.m.," Ricky Jensen, the school district's resource-conservation manager, said. "The temperature recovery is almost immediate, taking only 15 to 20 min. Other sites take three or four hours to heat up."

Nina Wolgelenter is a senior writer for Big Ass Fan Co. With a background in environmental education and journalism, her work is published frequently by magazines, newspapers, and online media outlets focusing on energy conservation and sustainability.

Did you find this article useful? Send comments and suggestions to Executive Editor Scott Arnold at [email protected].