A test program is underway at the Oak Ridge National Laboratory (ORNL) to evaluate the potential performance improvements from using nonazeotropic refrigerant mixtures in residential heat pumps. The goals of the research effort are to achieve (1) capacity control by adjusting the composition, (2) increased capacity, relative to R22, at low ambient temperatures, and (3) matching of the refrigerant temperature differentials to the source and sink fluid temperature differentials. Performance improvements are expected to result from decreasing the cycling losses and the amount of resistance heat required at low ambient temperatures along with reducing the irreversibilities in the heat exchangers. Initial results for a mixture of R13B1/R152a show a potential capacity modulation from 12431 Btu/h (3643 W) at 17 F (/minus/8.3/degree/C) to 8550 Btu/h (2506 W) at 47 F (8.3/degree/C) in heating and from 7451 Btu/h (2184 W) at 82 F (27. 8/degree/C) to 9188 Btu/h (2693 W) at 95 F (35/degree/C) in cooling. Other refrigerant pairs which are safer from an ozone depletion standpoint are under consideration for later testing. 10 refs., 5 figs., 4 tabs.