The relevance of the research is explained by the need to improve the reliability and to reduce the costs of repairing convective heat transfer surfaces in a boiler unit burning sulfur coals. The main aim of the study is to conduct optimization research and to compare different methods for air preheating before boiler units to reduce the rate of low-temperature corrosion of the air heater surfaces. The methods used in the study: thermal (verification and design) and optimization calculations on a mathematical model of the boiler unit BKZ-420-140-6. The model is supplemented by the flow diagrams of air preheating. The mathematical model of the boiler unit is developed using a system for computer-aided program generation (SCAPG-PC). The System was developed at the Energy Systems Institute SB RAS on the basis of applicable normative documents, technical feature of the boiler unit, instructions for its operation and results of its tests. The results: The authors have developed the mathematical model of the boiler unit which includes the calculation of a low-temperature corrosion rate, and makes it possible to define rather accurately the processes that occur in the boiler, and to determine the characteristics and the parameters of its operation. The optimization, technical and economic studies of the air preheating methods have been conducted. The findings allow the efficiency of the methods to be compared within the problem of the reduction in a low-temperature corrosion rate.