A simple analytical model to predict the onset of Ledinegg instability in vertical channels under downflow conditions has been developed and evaluated. The model divides the flow field into three regions based upon the fluid temperature. The pressure drop is then found by solving an appropriate set of equations for each region. The theoretical results are compared to an extensive set of experimental data covering a range of channel diameters and operating conditions. Agreement is excellent, and the prediction of the velocity at which the minimum point in the demand curve occurs is within 12 percent over the range of experimental results. A parameter, the ratio between the surface heat flux and the heat flux required to achieve saturation at the channel exit for a given flow rate, is found to be a very accurate indicator of the minimum point velocity.