A numerical study of laminar steady natural convection induced in a two dimensional horizontal channel provided with rectangular heating blocks, periodically mounted on its lower wall, is carried out. The blocks' surface temperature, T'(H), is maintained constant and the former are connected with adiabatic surfaces. The upper wall of the channel is maintained cold at a temperature T'(C) < T'(H). Fluid flow, temperature fields and heat transfer rates are presented for different combinations of the governing parameters which are the Rayleigh number (10(2) <= Ra <= 2 x 10(6)), the blocks' spacing (1/4 <= C = l'/H' <= 1), the blocks' height (1/8 <= B = h'/H' <= 1/2) and the relative width of the blocks (A = (L' - l')/H' = 1/2). The results obtained in the case of air (Pr = 0.72) show that the flow structure and the heat transfer are significantly influenced by the control parameters. It is found that there are situations where the increase of the blocks' spacing leads to a reduction of heat transfer. (C) 2009 Elsevier Ltd. All rights reserved.