Abstract <p>The results of numerical 3D modeling of free-convective heat-transfer processes in a small-sized “warm baseboard”-type heating systems are reported. The thermal and hydrodynamic parameters of the heated air flow are calculated for the vertical channel of the baseboard system, which is formed by two horizontal hot tubes with transverse ribs. The criterion dependences of the effective coefficient of heat transfer from the main structural and mode parameters of the system to air being heated are obtained. It is shown that due to the presence of tubes in the channel between the ribs, the heat-transfer coefficient that can be calculated using the found correcting factor decreases. The optimal values of the gap between the ribs and heating tube diameter, which ensure the maximal specific (per unit length) heat flux from the surrounding medium, are determined. Comparison of the calculated data with experimental results shows their satisfactory coincidence. The obtained results can serve as the basis for developing the methods for designing such heat-exchange heating systems. It is shown that the proposed optimal geometrical parameters of the baseboard system ensure a considerable increase in the heat power as regards the maximal values (by 30–40%) and in the possible range of variation of the specific heat power of baseboard system as compared to the known variants of such systems.</p>