The statistical data show that the application of active cooling is spread widely in residential and commercial buildings. In these buildings, the ventilation is significantly increased in the whole energy consumption. There are similar problems in the operation of post-insulation of existing buildings. In this case, the energy consumption of the ventilation system gives a major proportion of the whole building services energy consumption. The opportuneness of this research shows that the actual available calculation procedures and technical designing data are only rough approximations for analyzing the energy consumption of air handling units and the energy saved by the integrated heat or energy recovery units. There are not exact methods and unequivocal technical data. In previous researches, the production and development companies have not investigated the effectiveness of the energy recovery units under difference ambient air conditions and the period of defrost cycle when the heat recovery can only partly operate under difference ambient air temperatures. During this term, a re-heater has to fully heat up the ambient cold air to the temperature of supplied air and generate the required heating demand to provide the necessary indoor air temperature.