An improved version of the computer code THEAP-2, suitable for calculation of low flows (G < 50 kg/m2·s−1), is developed. The original code failed to provide reasonable agreement with existing experimental data. The discrepancies were attributed mainly to the drift-flux model, the dispersed flow transition criterion, and the correlations for estimating critical heat flux and minimum film boiling temperatures employed in the original code. The Electric Power Research Institute drift-flux model was used to correct these shortcomings and a new dispersed flow transition criterion was proposed. A review and an assessment of the available correlations of the temperatures resulted in the development of revised versions of these correlations. The changes improved the code’s ability to predict quantities such as the wall and vapor temperatures, the actual quality, and the vapor generation rate. The improvements can be attributed to the transition criterion introduced in the revised code.