An extensive series of measurements on wall-to-bed heat transfer coefficients, solids concentrations and solids velocities has been performed in a semi-industrial scale CFB unit running with a variety of solids fractions at ambient temperatures. Experiments have shown a significant influence of solids concentration and solids movement in the vicinity of the heat transfer surface on the wall-to-bed heat transfer coefficient. A model is proposed to describe the wall-to-bed heat transfer that is applicable over the entire range of possible local solids concentrations occurring in CFB systems.