An improved implementation of the Newton-Raphson method for the static analysis of fluid networks has been developed. This now allows circuits containing three and four terminal non-linear devices to be analysed. One type of multi-port device that is used to control ventilation circuits is the vortex amplifier. This is a three terminal (supply, control and outlet) controller with no moving parts. Vortex amplifier characteristics have been integrated into a ForTran steady state network analysis program which has been used successfully for designing ventilation systems controlled by these devices. Any form of pipe and device resistance can be defined. Experience of using this program has shown that the geometry of the network and the relative positions of high and low resistance pipework and devices can prevent numerical convergence. A more detailed examination of this has provided two methods of overcoming this non-convergence.