In order to simulate the behavior of heat pumps, theoretical models of heat exchangers and compressors are developed. The distribution of refrigerant flow rate between two paths in the heat exchangers can be satisfactorily simulated. Unbalanced distribution of the refrigerant flow rate among multiple paths frequently occurs in the condenser in contrast with in the evaporator. This different feature is ascribed to the different orders of magnitude of frictional and gravitational components of pressure drop in the condenser and the evaporator. The new compressor model requires neither the polytropic exponent nor the volumetric efficiency. The present models can estimate the mass of refrigerant existing in the heat exchangers and the compressor.