A semi-empirical approach, utilizing both basic heat transfer theory and the data of others (to estimate heat transfer coefficients) is used to develop the equations necessary to predict the points of onset of regular low-frequency oscillations. The equations lead to curves of condensing length as a function of coolant inlet temperature with mass flow rate ratio as the parameter. In addition to predicting the coolant temperature at which instability occurred, the model also predicted the condensing length (about 3 inches or 7.6 centimeters) at which oscillations began, thereby enhancing credibility of the model.