In the operation of solid fuel boilers the main problem is the fouling of their heating surfaces by ash deposits and slag. The intensity of fouling process is estimated by the amount of mineral matter contained in the fuel and its chemical mineralogical properties, also by the character of combustion regime and the regime of cleaning heating surfaces. The problems of the furnace thermal operation of oil shale boiler are discussed. The influence of the emissivity of furnace wall on its thermal efficiency is shown and some data on the emissivity of initial ash deposits formed in burning of shale oil with ash content over 0.5 % are given. The burning of oil shale or ash containing heavy oil produced from oil shale causes the decreasing of the thermal efficiency of boiler furnace and this phenomenon cannot be fully removed by intensive cleaning. Due to this, the output power of boilers using oil shale or ash containing shale oil may be lower than designed.