The application of the exergy approach to thermal comfort models is a modern tool for providing and evaluating the level of thermal comfort. The purpose of the paper is to apply exergy analysis to the “heat source - human - building envelope” system to reduce energy consumption and to identify the most influential factors. The regression models for determining comfortable indoor temperature taking into account subjective and objective microclimate parameters and indicators are obtained. The variety of influence factors, namely human activity, thermal resistance of clothing and the values of reference state for exergy evaluation, have been expanded for analysis within cold and warm periods. The paper proposes to investigate the effect of reference environment on both the exergy consumption of human body and the comfortable indoor air temperature. The changes of exergy consumption by the human body for various values of the reference environment and the subjective parameters of thermal comfort are estimated. It has been found that the decrease of the ambient temperature results in an increase of the exergy consumption in within the human body. However, the value of the comfortable air temperature in the room practically remains unchanged. Therefore, when considering the “heat source - human - building envelope” system, where the influence of a person is determined by the comfortable air temperature in the room, the reference environment can be chosen equal to the ambient temperature. Based on the obtained regression equations both the influence of thermo-modernization and the change of the building category for providing comfortable conditions are estimated. Dynamic mathematical models for representative samples of rooms with different design parameters (orientation, geometry) and thermophysical properties of enclosures have been developed. The models provide a possibility to evaluate energy performance and saving in case of determining comfortable indoor temperature based on the proposed regression model.