This publication is a study of the dynamic performance of a 5-ton air-to-air heat pump in a residence in Washington, D.C. The effect of part-load operation on the heat pump's cooling and heating coefficients of performance was determined. Discrepancies between measured performance and manufacturer-supplied performance data were found when the unit was in the heating mode and outside temperatures were below 40 degrees F presumably due to frost build-up on the outdoor coil. The seasonal performance factor (SPF) of the heat pump was estimated and traced back to the power plant to determine an "effective SPF" which was compared to the performance expected from fossil-fuel heating equipment. If the estimated seasonal performance is representative of residential situations in this type of climate then no clear-cut advantage in saving primary source energy exists in choosing either heat-pump or fossil-fuel heating system. This study is part of the National Bureau of Standards Building Science Series. (Author/MR)