Abstract In this paper, design charts for the evaluation of thermal parameters for natural convection with air in a channel–chimney system are proposed. In the thermal analysis of natural convection in channel–chimney systems, the variables that play an important role are: the ohmic heat flux, maximum wall temperatures and geometrical parameters such as the height of the heated channel, the channel spacing and the height and spacing of unheated extensions. A simple numerical procedure to obtain the thermal design charts, a thermal optimization of the system and an uncertainty analysis due to the thermophysical properties are presented. Results are carried out for symmetrically and asymmetrically heated channels with walls at uniform heat flux and a simple estimation procedure is proposed to evaluate the error in the relevant geometrical and thermal parameters due to a different value of the reference temperature. The estimated error, however, is less than the uncertainty of the experimental data employed. Some simple examples are given to show the use of the charts. The proposed results are obtained from experimental data in the following dimensionless parameter ranges: 5.0⩽Lh/b⩽20; 1.5⩽L/Lh⩽4; 1⩽B/b⩽4; 102⩽Ra⩽106.