In the present study, a comparative investigation has been performed on the impact of the hot gas passages on a sectional cast-iron boiler (CIB) performance. To this end, various shapes of the hot gas passages such as the circle (case-1), hexagon (case-2), square (case-3), vertical ellipse with constant aspect ratio (AR) of 2 (case-4), and horizontal ellipse with AR=2 (case-5), have been considered and compared with each other in order to find the best geometry among the cases under consideration. Computations are carried out at two water flow rates of 0.015 and 0.030 kg/s and a three-dimensional flow domain is designed based on the dimensions of a real CIB section. It is found that case-5 has the maximum heat transfer rate and goodness factorwith the highest sensitivity to the water flow rate among all cases under consideration. It was hoped that the obtained results arouse interest among the boiler designers.