The base coil had a uniform fin density of 8 fins/cm on the three rows. The fin staged coil had a fin spacing of 6 fins/cm, 8 fins/cm, and 10 fins/cm on the front, second, and back rows, respectively. Frost/defrost performance was evaluated at -2.2 °C with 90% relative humidity and at 1.7 °C with 82% and 95% relative humidity. Two airflow rates were tested. Frost growth on the leading edge of the front row was measured. These experiments showed dependence between outdoor conditions, airflow, fin geometry, and heat pump performance. Decreasing airflow by 25% reduced cycle time by approximately 35%, yet it had no measurable effect on the cyclic COP. Further research should investigate other refrigerant circuiting schemes as well as other fin densities and configurations and other airflow rates to help localize the optimum staging for the heat pump.