The contribution of hydrofluorocarbons to global warming has led to the reinvestigation of hydrocarbons (HCs). Despite the flammability of HCs, some manufacturers of refrigerators and residential air conditioners (especially in the European Union and Japan) have begun to use HCs as refrigerants in small capacity equipment.1,2 The increased use of HCs in refrigerators brings up the applicability of HCs in residential air-conditioning units and heat pumps. Among the HCs, R-290 has a similar vapor pressure to that of R-22. To contribute to a clearer understanding of the relative performance potential of R-290 as compared to R-22, the performance of both refrigerants was measured according to ASHRAE standards3,4 in a residential, state-of-the-art, split-system air-conditioner heat pump using psychrometric chambers at the Center for Environmental Energy Engineering at the University of Maryland.