The unauthorized changes to heating system of multistorey residential building pose an actual problem at present. These changes often cause a serious consequences or can destroy the system.The most common type of heating systems in mulytistorey buildings is one-pipe system. It is the most simple type, and the impact of the changes in these systems is bigger, than in another one (for example, two-pipe). In this article tipical unauthorized changes to one-pipe heating system were considered. The most popular chages are replacement of heating devices and pipes, changing the size and the material. The impact of these changes was analysed.