An experiment has been performed to investigate the natural convection heat transfer coupled with the effect of thermal conduction from a steel plate with discrete heat sources. Two boundary conditions, with constant heating flux and constant heating temperature, were obtained by controlling the heating flux or temperature of the heat sources mounted on the back of the plate. The heat transfer and temperature distribution of the plate were measured with different heating spaces. By defining the average heating temperature of the plate as the characteristic temperature, simple expressions for the average heat transfer are correlated from the experimental data as a function of the relative heating space and Rayleigh number. The error is 5% in the Rayleigh number range from 5 x 10(8) to 5 x 10(9). (c) 2006 Elsevier Ltd. All rights reserved.