An air-to-air heat pump was installed in an unoccupied single-family residence and operated in a two-year test to characterize dynamic losses in capacity and efficiency due to cycling, frosting, and defrosting. During the heating season, defrosting losses were responsible for 10.2% of the total energy consumption (excluding supplemental electric resistance heating), frosting losses for 3.7%. start-up transient losses for 8.5%, and off-cycle parasitics for 3.3%. The heating cyclic degradation factor, C_d, was estimated to be 0.26. Cooling mode steady-state performance of the heat pump was degraded from the manufacturer's ratings due most likely to a small refrigerant leak resulting in a slight charge deficiency. However, using the site-measured performance as the steady-state base, it was found that start-up transient losses accounted for 2.8% of the total energy use and off-cycle parasitics for 4.4%. The cooling C_d was estimated to be 0.11 on this basis.

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