Temperature offsets from elevated air speeds can be obtained by setting air temperature at 77°F. The other reference settings for Thermal Comfort Tool are:

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  Relative humidity: 50 percent.

• Metabolic rate: 1 met.

• Clothing insulation: 0.5 Clo.

• Air speed: 30 fpm.

The dry-bulb-air-temperature setting, then, is increased until the PMV is +0.5, the upper limit of the comfort zone to satisfy 80 percent of occupants. The difference between 77°F and the temperature setting at a PMV of +0.5 is the temperature offset.

ESTIMATING COOLING-ENERGY SAVINGS FROM TEMPERATURE OFFSETS

Facility-maintenance engineers at Travis Air Force Base in California claim that turning up the thermostat on air-conditioning equipment 2°F reduces cooling operating costs by 8 percent (4 percent per degree Fahrenheit).⁵ Using a savings rate of, say, 3 percent per degree Fahrenheit to allow for fan energy on a 4.7°F temperature offset provides a net savings of 14 percent.

In spaces characterized by non-sedentary activity, such as gymnasiums, elevated air speeds of up to 315 fpm can provide temperature offsets of up to 6.9°F. The potential energy reduction from a 6.9°F temperature offset using a savings rate of 3 percent per degree Fahrenheit is 21 percent. A 24-ft-diameter ceiling fan, operating at a speed of 30 rpm using 0.63 kw of electricity, can provide a mean air speed of 315 fpm near floor level over an area of 7,844 sq ft (0.08 w per square foot).

DESTRATIFICATION SAVINGS DURING COOLER MONTHS

When a space is heated during cooler months, hotter air rises toward the ceiling, stratifying with a significant temperature gradient — typically, 0.75°F per foot from floor to ceiling. In a space over 10-ft high, heat energy above head height is wasted, as it does not contribute to occupant thermal comfort in the occupied zone. Thorough mixing of this hotter air with cooler air (destratification) results in a uniform air temperature throughout the space.⁶ Typically, 10 percent of heating energy is saved for each 10 ft of floor-to-ceiling height.

Approximately half of the air in a space needs to be circulated every hour for effective destratification. Twenty-four-foot-diameter circulator fans set at low speed (6.9 rpm) offer much greater aerodynamic efficiency (up to 669 cfm per watt) than smaller fans, delivering 98,940 cfm. Five-foot-diameter fans operating at 300 rpm (301 cfm per watt) deliver 43,878 cfm.

DESTRATIFICATION IN AIR-CONDITIONED SPACES

HVAC engineers often have difficulty balancing air-conditioning systems. Large circulator ceiling fans can mix the air in air-conditioned spaces to a uniform temperature and eliminate system-balancing problems. Large circulator ceiling fans also can serve as the first method of cooling in milder climates, delaying the startup of an HVAC system for substantial energy savings.

CONCLUSION

Energy consumption will be a concern of business owners as long as heating and cooling costs continue to fluctuate. The need to control costs, however, must be balanced with the need to maintain comfort, thus, ensuring high productivity and occupant satisfaction. With large circulator fans, higher rates of efficiency can be achieved than with HVAC systems alone. In some cases, large circulator fans even can eliminate the need for air conditioning. When air conditioning is required, however, large circulator fans can reduce the load on an HVAC system and eliminate the need for ductwork. With proper air movement, desired occupant comfort and reduced energy consumption can be achieved for any space.

REFERENCES

1. Aynsley, R. (2007, November). Circulating fans for summer and winter comfort and indoor energy efficiency (vol. 1: TEC 25). BEDP Environment Design Guide. Melbourne, Victoria, Australia: Australian Council of Built Environment Design Professions.

2. ASHRAE. (2005). ASHRAE handbook-fundamentals. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers.

3. De Dear, R.J., & Brager, G.S. (2001). The adaptive model of thermal comfort and energy conservation in the built environment. International Journal of Biometeorology, 45, 100-108.

4. Koenigsberger, O.H., & Lynn, R. (1965). Roofs in the warm humid tropics. London: Lund, Humphries.

5. Feldpausch, N. (2006, June 2). Efficient thermostat settings saves energy, base money. Air Force Print News Today. Retrieved from http://public.travis.amc.af.mil/news/story_print.asp?storyID=123021158

6. Aynsley, R. (2005). Temperature profiles and winter destratification energy savings. Ecolibrium, 4, 18-23.

Richard Aynsley, PhD, is director of research and development for Big Ass Fans. Over the last 38 years, he has conducted research and consulting in the areas of building science, energy efficiency, and building aerodynamics for commercial and government clients in the United States, Australia, and New Zealand. He has a bachelor's degree in architecture from The University of New South Wales (UNSW) in Sydney, Australia; a master's degree in architectural engineering from The Pennsylvania State University; and a doctorate in bluff-body aerodynamics from UNSW.