Model reduction techniques are often applied to improve the computational speed when finite volume models are used to describe the dynamics of heat exchangers (HXs) in vapor compression systems. One particular area of focus has been the simplification of the momentum equation, since it usually does not affect the thermal system behavior over the time scales of interest. This paper compares the effect of five reduced order variants of the momentum equation on the dynamics of complete vapor compression cycles. This comparison is achieved via the implementation of a dynamic model of an air-source heat pump, which enables the direct comparison of these different approximations on the system dynamics, prediction accuracy, and simulation speed on otherwise identical models. Simulations indicate that neglecting the phenomena on small time scales can greatly improve numerical efficiency with a minimal loss in prediction accuracy. The variants with frictional pressure drop only and with linear pressure distribution are also found to outperform other variants in both prediction accuracy and simulation speed.