A residential air-conditioning system using an expansion valve experiences a higher coefficient of performance (COP) at reduced ambient temperatures than one employing a capillary tube as the expansion device. This study explored the effect of this improved performance on the seasonal COP of two air conditioning systems that were identical except for the expansion device. Using the 1974 weather in Chicago the seasonal COP of the expansion valve system was 2% higher than of the capillary tube system. An air conditioner using an expansion valve in Houston with 1966 weather showed a 3% higher seasonal COP than the capillary tube system, because much of the cooling operation in Houston occurs at mild ambient temperatures.