The use of computational fluid dynamics for simulation of combustion processes has made significant advances in recent years particularly for the design of individual burners and the prediction of pollutant formation and emission. However, the computational requirements of these models can still be too great for overall furnace thermal design purposes particularly if the transient performance is required. Thermal radiation is usually the dominant mode of heat transfer to the load or stock in industrial fuel-fired furnaces since the contribution of convection is relatively small. Thus prediction of the thermal performance of a furnace requires an accurate calculation of the complex radiation interchange between the surfaces and the combustion products. This can be achieved by the so-called Hottel zone method of radiation analysis and as a result this method has been applied to a wide range of industrial heating processes. The method sub-divides the non-isothermal furnace enclosure into a series of isothermal volume and surface zones and energy balances are then formulated and solved simultaneously for each zone. The computational demands are modest so that the process can be repeated successively throughout a period of furnace operation to simulate the transient behaviour of the system. However in these models all the surfaces are usually assumed to be grey and the radiation properties of the combustion products are normally represented by a mixture of grey and clear gases. These assumptions can lead to errors in the predictions, in applications such as the installation of high emissivity coatings on the furnace lining, where it is necessary to allow for the spectral variation in surface emissivity and the banded nature of the radiation properties of carbon dioxide and water vapour in the combustion gases. Consequently the proposed paper describes the development of “spectral” zone model, which takes these effects into account, to predict the transient performance of a furnace heating steel bars to a discharge temperature of 1200°C. The model also allows for broadening of the spectral bands with changes in the temperature of the combustion products. The work differs from that in previous papers on this type of model, which have been confined to steady-state simulations and do not allow for broadening. Finally the model is applied to investigate the effect of coating the refractory lining of the furnace with high emissivity materials.