A detailed but still central processing unit (CPU)-efficient bed model for grate-fired combustion of biomass and waste is developed. Simulations of wood chip combustion are performed, and the results are compared to experiments. The so-called layer model is used to track the development of the thermally thick representative fuel particles in the bed. As an efficient way of handling a large number of physical fuel particles, each representative fuel particle represents a number of physical particles with the exact same properties. The motion of the fuel bed is handled in a way that requires negligible CPU power, while for wastes and other fuels with less defined shapes and structure, it still yields accuracy similar to the much more CPU-intensive collision-based models. In this work, the bed model is coupled with ANSYS Fluent and used to simulate one of the test campaigns performed at the grate-fired pilot unit at the University of Stuttgart. It is found that for the test campaign of interest, burning wood chips, the fuel bed is ignited from below, and it is explained how this is due to the thermal properties of the grate and how important the numerical handling of the grate is for an accurate prediction of the bed behavior.