Florida Energy Extension Service Whole House Fans: Benefits and Description 1 Gary D. Cook 2 In Florida s mild climate whole house fans offer an excellent opportunity to save energy and money when compared with the alternative of using mechanical air conditioning. To reap the maximum benefit from a whole house fan, attention should be given to proper sizing, installation, and operation of the fan. BENEFITS The whole house fan is an energy efficient substitute for air conditioning during certain portions of the day and year when outside conditions are favorable. In most areas of Florida, there are about 120 days when the whole house fan may be used effectively. A three ton air conditioner typically used to cool a 1500 square foot (SF) home draws about 5 kilowatts of power. A 30 inch whole house fan moving about 5100 cubic feet per minute (CFM) of air draws less than 0.5 kilowatts of power. At 7.5 cts a kilowatt hour (Kwh) that translates to a savings of $.34 (4.5 kilowatts x $.075) for each hour that the whole house fan is substituted for the air conditioner. A properly selected, installed, and operated fan can reduce overall cooling costs by as much as 50%. Whole house fans cost about as much as a good ceiling fan ($150-300). Savings usually pay for the fan in less than three years. The whole house fan enables Florida residents to achieve positive benefits because of four characteristics: 1) It produces a wind chill effect f o r t h e occupants making higher temperatures (82-88 degrees) feel just as comfortable as lower temperatures (74-78 degrees) without air movement. 2) By flushing the house with high volumes of outside air, it keeps temperatures from building up on the inside from internal and radiant loads, thus delaying the need for the air conditioner. 3) It reduces the temperature of the attic by about 10 degrees F on a warm summer day. This significantly reduces the mean radiant temperature (MRT) of the ceiling. radiant temperatures have a high impact on our comfort level. 4) A whole house fan can quickly remove pollutants within the house such as cigarette smoke, cooking odors and harmful household cleaner and spray emissions. These benefits can be optimized by proper selection, installation, and operation. For example, operating the whole house fan during periods of very 1. This document was published as Fact Sheet EES-32, Florida Energy Extension Service. For more information, contact your county Cooperative Extension Service office. 2. Energy Extension Specialist for Building Construction, Florida Energy Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville. The Florida Energy Extension Service receives funding from the Governor s Energy Office. The information contained herein is the product of the Florida Energy Extension Service and does not necessarily reflect the views of the Governor s Energy Office. The Institute of Food and Agricultural Sciences is an Equal Opportunity/Affirmative Action Employer authorized to provide research, educational information and other services only to individuals and institutions that function without regard to race, color, sex, or national origin. Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences / University of Florida / John T. Woeste, Dean
high humidity may be counterproductive even if outside temperatures are moderate. There are three factors that affect comfort: air movement, temperature of adjacent surfaces, and relative humidity. Generally, a lightly active person will feel comfortable in surroundings with surface temperatures averaging 78 degrees F and still air at 78 degrees F and 50% relative humidity. Moderate air movement will enable a person to feel comfortable with air temperatures as high as 90 degrees F and 50% relative humidity (see Table 1). However, a fan should not be used when the outside air temperature is more than 85 degrees F. Fan use during this period will only draw hot air into the house increasing heat build up in walls, floors, and furniture. Higher surface temperature radiation might then offset the wind chill factor. OPERATION The operation of a whole house fan is relatively simple. Use the fan when the outside air temperature and relative humidity conditions can produce a wind chill or perceived temperature that is lower than the inside of the house. Generally, the warmest part of the day occurs around 3pm and the coolest part of the day is around 6am. Unfortunately, the most humid part of the day generally occurs early in the morning when temperatures are coolest. The exception to both is when frequent afternoon thunderstorms occur during Florida summers. s then drop and relative humidity rises to 100%. Whole house fans should not be operated during periods of rain or other high humidity conditions. At these times, the fan will draw in moist air allowing books and materials made of wood and fabric to absorb moisture. This can contribute to mold and mildew problems and a higher load on the air conditioner when used. A general rule is not to use the fan when the relative humidity exceeds 85%. A combination thermometer/humidistat would prove a wise investment to aid in the decision of when to operate the fan. HUMIDITY Although outside temperatures in Florida can exceed 100 degrees F, it is rare to have temperatures greater than 94 degrees F. South Florida coastal areas rarely get temperatures over 90 degrees F although humidity levels are generally higher. temperatures and estimated mean relative humidity levels in various locations in Florida are provided in Table 2. This table shows there is ample opportunity to use the whole house fan throughout Florida even in the summer season. POLLUTANTS Whole house fans can be used to remove indoor pollutants such as household cleaning product fumes, formaldehyde emissions, hydrocarbons, insecticide fumes, smoke, nitrogen dioxide, and carbon monoxide from smokers and gas cooking and chlorine from bleach. By moving a high quantity of outside air through the house, a whole house fan can clear a smokey home in less than five minutes. It would take the average small air cleaner more than 24 hours to do the same job. Prudent use of the whole house fan is recommended during periods of high outside pollution levels. Local weather reports generally report air quality indices. Nearby construction may produce more than the average amount of airborne dirt. Under average conditions, the use of the whole house fan will not increase dust in the house. In fact, there is some evidence to suggest it will contribute to a cleaner house because air movement will not let fine dust settle. If an air conditioner is to be used with the whole house fan, their operation should be coordinated for maximum benefit. Never use the whole house fan and the air conditioner at the same time except to clear the house of pollutants such as smoke and odors. In summary, the proper selection, installation, and operation of whole house fan can provide many extra
Page 3 Table 1. Weather Data for Florida Locations Weather Data for Florida Locations Location Months Estimate Humidit y Rainfall Inches Gainesville May 76 deg F 63% 3.54 June 80 deg F 69% 6.81 August 81 deg F 73% 8.25 September 79 deg F 73% 5.67 Jacksonville May 74 deg F 70% 3.22 June 79 deg F 73% 6.27 August 81 deg F 73% 7.89 September 78 deg F 79% 7.83 Key West May 81 deg F 68% 2.51 June 84 deg F 70% 4.55 August 85 deg F 72% 4.47 September 83 deg F 72% 7.34 Miami May 78 deg F 70% 6.12 June 81 deg F 73% 9.00 August 83 deg F 75% 6.72 September 82 deg F 78% 8.74
Page 4 Table 2. Weather Data for Florida Locations, Continued Weather Data for Florida Locations Location Months Estimate Humidity Rainfall Inches Orlando May 78 deg F 63% 2.94 June 81 deg F 69% 7.11 August 83 deg F 72% 6.73 September 81 deg F 73% 7.20 Pensacola May 75 deg F 71% 4.25 June 81 deg F 69% 6.30 August 82 deg F 77% 6.67 September 78 deg F 70% 8.15 Tampa May 77 deg F 69% 2.41 June 81 deg F 69% 6.49 August 82 deg F 74% 8.00 September 80 deg F 77% 6.35 Source: National Oceanographic and Atmospheric Administration
Page 5 hours of comfort by drawing in relatively mild outside air and removing hot, stale air from a home by pushing it through the attic, cooling it in the process. Cooling costs can be reduced by as much as 50% compared to using an air conditioner alone. BIBLIOGRAPHY 1. Cook, Gary D., "A Guide To Residential Energy Efficiency in Florida," EES 7, Energy Extension Service, Institute of Food and Agricultural Sciences, University of Florida in cooperation with the Governor s Energy Office, 1981. 2. Cook, Gary D., "Design, Marketing and Operation of Energy Efficient Buildings," Energy Extension Service, Institute of Food and Agricultural Sciences, University of Florida in cooperation with the Governor s Energy Office, 1982 (limited edition). cooperation with Solar Energy and Energy Conversion Laboratory, University of Florida. 4. Lafavore, Michael, "Something s in the Air," New Shelter Magazine, Rodale Press, Emmaus, Pennsylvania, May/June 1982. 5. New Shelter s Testing Department, "A Test of Small Air Cleaners," New Shelter Magazine, Rodale Press, Emmaus, Pennsylvania, July/August 1982. 6. Spingler, John D., Sexton, Ken, "Indoor Air Pollution -- A Public Health Perspective," Science Magazine, Vol. 221, No. 4605, July 1983. 7. Tiedemann, Thomas, Brown, James, Nawrock, Paul, " Solar Radiation for Florida Cities," FSEC-77-3, Florida Solar Energy Center, April 1977. 3. Farber, E.A., Wiggins, D.B., Podris, F.I., Murdoch, J.A., "Attic Ventilation," Technical Report, Florida Governor s Energy Office in Chill Effect of Air Moving at 300 Feet Per Minute (3.4 MPH) Actual s Humidity Sensed Chill Effect 82 deg F 73% 71 deg F 11 deg F 85 deg F 75% 74 deg F 11 deg F 85 deg F 64% 72 deg F 13 deg F 90 deg F 65% 78 deg F 12 deg F 90 deg F 50% 76 deg F 14 deg F