The complex technical object usually can be described as a set of technological components (blocks). Its functional diagram can be represented by an oriented multi-graph. The vertices of the graph correspond to separate components (blocks) of a technical object. The blocks are characterized by design parameters. The graph may contain feedback loops. Blocks’ output parameters can be determined from the values of their input parameters. The software developed in C++ language serves for calculating functional diagrams of steam boilers. The software determines also the values of derived output parameters from the design parameters values. It can be used to improve the technical characteristics of an object and determine control parameters for various modes of its operation. The calculation of the thermal scheme is based on the use of a model heat exchanger. In this case, the calculation of the thermal scheme is reduced to an iterative solution of a system of linear equations. A brief description of the software is provided.