This paper deals with the topic of heat transfer. Part of the project is devoted to describing the basic theory of heat transfer. After short introduction into the topic by clarifying the concept of heat in general, attention is directed to methods of heat transfer: conduction, convection and radiation. Conduction is a transfer of heat between particles (atoms, molecules, electrons) as a result of their interactions. Convection is a method of heat transfer between a solid surface and the surrounding fluid in motion. Thermal radiation is the form of radiation emitted by each body with a temperature higher than the absolute zero. The concept of thermal resistance is also mentioned as something various mediums provide against heat transfer. The application of heat transfer is shown by the example of heat exchange between the heated space and the environment. The “KI Expert Plus“ software is used to solve the problem. The calculation is done for the residential unit located in a city of Pula by following the standard HRN EN ISO 13790. The software requires data input such as surface area of the apartment, surface area and type of building material, windows surface area, thermo-technical system specifications, etc. Based on the entered data, the software calculates heat gains and losses. The project goal was to rank the apartment in the energy class.