Thermal, mechanical and chemical components of room air exergy and its change depending on the outside air parameters (temperature, humidity and pressure) have been examined for different periods of the year, using the climatic data for Kiev (IWEC). In winter the thermal component dominates, whereas in summer the chemical one prevails. Various approaches to exergy efficiency definition of ventilation systems with energy recovery (namely, a universal, functional and maximum exergy efficiency) have been analyzed at different boundaries of the system (in a heat exchanger and in in an installation at large). Dependence of these parameters on the temperature and moisture content of outdoor air (reference environment) was studied. In addition, there have been calculated weighted average exergy efficiency coefficients for cold and warm periods of the year for the city of Kiev.