A heat balance over the fluid-dynamic boundary layer of a circulating fluidized bed furnace has been formulated in order to calculate the heat transfer to the walls. The convective heat transfer is related to a horizontal particle how from the core to the wall region, and an additive radiative heat transfer constituent is calculated for the particulate medium. An empirical description of the vertical distribution of solids and the transversal temperature profile are needed for the evaluation. The calculated heat transfer coefficients were compared with measurements in three commercial boilers and the deviation was less than 40%, which is in the same order as the uncertainty of the experimental data.