INTRODUCTION The performance of residential and commercial airconditioners has a large influence on electrical energy consumption in the United States. In 2004@ households in the U.S. used 216.8 billion kWh of electricity for air-conditioning@ accounting for 6.4 percent of all electricity consumed by residences. Commercial buildings in 2004 consumed an additional 169.9 billion kWh of electricity for air-conditioning. In 2001@ 80.8 million households within the U.S. were airconditioned. In 2003@ 3.6 million commercial buildings were air-conditioned. Most of those were in the Southwest@ south Atlantic@ Midwest@ and middle Atlantic states. The number of air-conditioners found in any locale can be attributed to two primary factors: population and climate. Manufacturers rate the performance of their products according to strict standards; however@ there are many factors that can affect air-conditioning equipment performance after installation. One common condition found in the field is dents within fins of condensers. There are various causes of dents; one is impact from hail; others include strikes from windborne debris@ and brushes of various items against the fins. Much of the continental United States experiences hail. Areas east of the Rocky Mountains@ particularly the southwestern and midwestern states@ are especially prone. In those areas@ warm moist air from the Gulf of Mexico collides with cold air from the north@ creating conditions that favor hail. Figure 1 is a map of the United States depicting locations of all reports of hail@ ? inch (19.1 mm) in diameter and larger@ from 1981 to 1990. Included on that same map is a breakdown by geographical region of the number of residences and commercial buildings with air-conditioning. From the map@ it is readily apparent that hail-prone regions have many air-conditioned buildings. Hail's effect on crops@ roofing@ automobiles@ and aircraft is well documented but there does not appear to be much understanding of its impact on air-conditioning equipment performance. As far as the authors are aware of@ no studies have been published regarding hail effects on air-conditioners@ specifically. A related study reported by Dooley indicated that fouling over time had minimal affect on efficiency and capacity. Other than the common practice of combing dents in repairing and maintaining air-conditioners@ there does not appear to be published data quantifying effects dents in fins have on performance. Hail physically alters air-conditioning systems by denting their condenser coil fins. Typically@ condenser coil assemblies comprise punched and pressed aluminum fins fitted onto copper coils. Fin spacing generally varies between 8 and 20 fins per inch (3.1 and 7.9 fins per cm). The thin cross sections of the fins serve well to conduct heat from the refrigerant in the coils and transfer it to air passing by them@ but these fins are easily bent. Figure 2 is a photograph of a condenser with fins deformed by hail. Not much has been reported on the extent air-conditioning system performance becomes degraded when fins are folded over in this way. A systematic study of the effect of fin dents on airconditioning system performance is performed. Hail creates a wide range of dent shapes@ sizes@ and depths in condenser fins. Dent characteristics are governed by many variables including properties of the impacting hailstones (size@ hardness@ and speed)@ orientation of the condenser to wind@ and shielding features surrounding the condenser. The present study does not fully replicate actual field conditions but is limited in scope in dealing only with a controlled@ laboratory situation to help in understanding the more complex denting scenarios in practice. The study does incorporate several defining aspects of dents caused by hail such as a random distribution of the dents@ a discrete size of each dent@ and a dent characterized by fins folded on top of one another. The limited goals of this study are to determine the effects that dents have on air-conditioning system performance and to determine to what extent conventional fin repair methods can restore performance.