Local values of heat transfer to membrane walls of a circulating fluidized bed (CFB) boiler were measured for three sizes of the same silica sand, with mean diameters of 0.22, 0.34 and 0.44 mm. A change from 0.44 to 0.22 mm sand at constant fluidization velocity led to a considerable increase in particle concentration and, hence, heat transfer. However, at a given cross-sectional average bulk density, the average heat transfer coefficient across the membrane wall was insensitive to the changes of bed particle size. Also, the lateral distribution of heat flow, to the crest and side of the tube and to the fin, was independent of particle size when the bulk densities were similar. This was achieved by keeping constant the ratio of fluidization velocity to terminal velocity of a single average size particle. It was possible to estimate the vertical distribution of the heat transfer coefficient in the CFB furnace with an accuracy of +/-20% by a simple semi-empirical method.