The most important thermophysical property for heat transfer through the ash deposits, accumulated at the waterwalls of the pulverized coal-fired furnaces, is the effective thermal conductivity. It affects the values of the wall variables: heat flux, temperature, and emissivity, and furnace temperature. In numerical simulations, the values of the effective thermal conductivity are used in combination with the ash deposit thicknesses. The objective of this investigation was to find the combined effects of the effective thermal conductivity and deposit thicknesses on the results of numerical simulations. Three curves were formed on the basis of the data available in literature. For every curve, three thicknesses of the non-uniform ash deposits were determined on the basis of the base normal distribution. The thickness of every surface zone was determined as a product of the number obtained from the diagram which shows dependence of the mean wall flux versus uniform thickness and corresponding thickness of the base distribution. The wall variables and flame temperatures were compared for the three values of the mean wall fluxes. The results showed that type of the curve did not influence the mean values of the wall variables and flame temperatures. The curves influenced the distribution of the wall variables very little. The maximal mean relative difference was obtained for the wall flux and it was less than 2%. The results indicates that the level of slagging can be defined by the ratio of the mean wall fluxes of the clean and wall covered by the ash deposit.