A model of fluid flow and heat transfer in planar flow casting has been developed, to examine the stability of the puddle formation process. Unlike such models developed previously, an applied pressure rather than an inflow rate is prescribed as the boundary condition at the nozzle inlet. A reference simulation is presented, followed by results that demonstrate the effect of varying operating conditions. Unlike the results of a previous fixed-inflow model, that yielded a wide range of steady-state results, this model yields solutions that illustrate the limited stability of the process. Results demonstrate the range of applied pressure that yields steady solutions. The nozzle/gap height is also shown to dramatically affect solution behaviour and stability, and results are also presented for changing wheel speed and heat transfer coefficient. Finally, the results are related to the so-called "operability window" proposed by Steen et al., as a means of describing the stability of the process.