The assessment of the influence of fouling and slagging on the heat transfer in utility boilers has obtained significant interest both during boiler design and operation. This paper presents a strategy to investigate this influence by introducing heat resistance to represent fouling and slagging on furnace walls. The evaluation of this strategy was performed using the plant heat flux reading data together with a 3-D computational fluid dynamics code. The model considers the influence of the operating conditions to the incident heat fluxes and these are used to assess the heat transfer resistance from the measured absorbed heat fluxes. Copyright (C) 2002 John Wiley Sons, Ltd.