Thermal calculations for furnaces with a circulating fluidized bed is made difficult due to the paucity of experimental data. This is based on the experience of operating only a few plants. As a prototype the boiler of a Lurgi plant is taken, including the furnace, cyclone and cooling chamber, i.e. the full complement of plant, some items of which may not be present in other systems. The authors discuss optimal selection of cyclones, material balance of circulating fluidized bed furnaces and calculation of heat transfer in the furnace.