A physicomathematical model making it possible to calculate thermal regimes of electric resistance furnaces has been proposed. The model is suitable for description of linings and heated products of different types. It includes, as components, the following models: those of thermal-radiation transfer in the furnace’s workspace, of nonstationary heating of target products, of nonstationary heating of the furnace lining, and of external heat exchange of the furnace’s enclosing structures. The distinctive features of a numerical method for solution of the proposed model are discussed. An example of calculation of a chamber electric resistance furnace for the cases where it is lined with fireclay brick and lightweight fibrous materials is discussed. It is shown that replacement of the lining by a fibrous one improves the thermal operating efficiency for this type of furnace 2–2.5 times.