Two dealer-supplied and -installed, commercially available air-to-air heat pumps were tested under both steady state and long-term field conditions at a site near Knoxville, TN. Results from these tests indicate that in the Knoxville area, during the heating season, losses caused by duty cycling, frosting, and defrosting may exceed 40% in some spring months and cause seasonal efficiencies to fall about 20% below the efficiency of steady state performance. Another approximately 5% decrease in efficiency results from resort to backup resistance heaters at low ambient temperatures. In the cooling mode, cycling losses reduce efficiency by about 10 to 20%.