Once a systems modeler knows the basic components in a refrigerant unit, the next task is to decide which variables or parameters to use in evaluating its equilibrium operation. The addition of a component as a simple maintenance convenience can mean a new set of convergence variables and resultant shift in final operating statepoints and system performance. This paper highlights some of the major criteria used in determining appropriate parameters to use in the convergence logic for modeling refrigerant units, specifically those used in HVAC service. Also included is a recommendation for an efficient matrix technique to be used in driving the system to convergence.