For this study, CFD numerical experiments to obtain Nusselt number correlation were examined. The condition of a residential heat pump system is used as a reference of inlet conditions fornumerical experiments. The thermo-dynamic state of CO2 in the condition passes the pseudo-critical point. Consequently, the temperature difference of the fluid between the inlet and wall is 2 C, sufficiently small to avoid the rapid change of fluid properties around the pseudo-critical point. From calculations changing inlet temperature and the flow rate, the Nusselt number correlations for
the water (cold) side and CO2 (hot) side can be obtained individually.A method to evaluate the heat transfer performance of the whole heat exchanger from the two correlations is proposed. The method through the new correlations is validated through comparison with experimental results using a test piece of the hot water supplier.