UF research: Undercharged A/C systems a common, costly problem

July 31, 2001

GAINESVILLE, Fla. --- As the dog days of summer continue, lowering your electricity bills while staying cool and conserving energy may be easier than you think.

Engineering researchers at the University of Florida have found that the performance of home and business air conditioners plummets when they are undercharged with coolant - and that the problem is both common and easily remedied. Most consumers would recover the cost of having their system charged in savings on power bills in three months, the researchers say.

"The moral of this story is that a lot of people could save money and conserve energy through a relatively simple and inexpensive procedure," said S.A. Sherif, professor of mechanical engineering at UF.

In an experiment at UF's Solar Energy and Energy Conversion Laboratory, Sherif and Yogi Goswami, a UF professor of mechanical engineering and the lead researcher, measured the performance of a 3-ton residential air conditioner charged with varying levels of refrigerant, according to a June paper in the International Journal of Energy Research.

The air conditioner worked almost normally when charged at the 90 percent level. But when the charge dropped below 85 percent, the performance declined significantly, with the system providing at least 15 percent less cool air than normal. At 50 percent charge, the researchers were unable to measure any cooling at all, they report.

The findings are notable because, the researchers discovered, many home and business air conditioning units are chronically undercharged.

As part of the study, they surveyed 22 randomly chosen commercial and residential air conditioners in Gainesville, Fla. A total of 17, or more than 75 percent, were undercharged at the 85 percent level or less. Surveyed units ranged in size from 3 to 10 tons and included a unit at a private residence, one at a fast-food restaurant and several at small office buildings.

"You don't have to go very far to find this problem," Goswami said. Undercharged A/C systems use more electricity than fully charged systems because they have to run longer to achieve the same cooling effect. Over time, the difference in electricity consumption can be quite large -- especially if the system is cooling a home or business in a warm, humid climate such as in Florida.

For example, the researchers estimated that a 3-ton residential system in Jacksonville, charged to 85 percent of its capacity, would use 8,830 kilowatt hours of electricity in a year. A fully charged system in the same climate would use just 3,885 kilowatt hours. "People think their system is working fine because they're getting cooling, but that's not the case if it runs too frequently," Goswami said.

A/C systems can lose their charge because of a major leak, a problem that may require replacement or a costly repair job. But systems more typically have tiny leaks that lose coolant very slowly. For example, it may take five years for some A/C units to lose 15 percent of their charge, Goswami said.

As a result, repairing an undercharged system often consists of little more than topping off the charge. The researchers estimated that such a routine maintenance job would require a maximum of two hours of labor and 1 pound of coolant. At $60 per hour for the labor, the consumer would pay approximately $130.

Depending on the climate, consumers would save that much money on their electricity in a short time, the researchers found. In Jackonsville, the payback period would be four months, while it would take about two and a half months in Tucson and a little over three months in San Antonio, they found.

Properly maintaining air conditioning systems also helps conserve energy, an increasingly important national priority, Goswami noted. "The current administration has recognized that energy conservation can pay back important dividends," Goswami said. "This is one way to achieve that goal."
The research was funded by a $34,500 grant from Florida Power & Light.

Writer: Aaron Hoover. Sources: Yogi Goswami, S.A. Sherif ahoover@ufl.edu 352-392-0812, 392-7821 goswami@ufl.edu,sasherfi@ufl.edu.

University of Florida

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