Renovation of existing buildings offers a great opportunity to reduce energy consumption, but often it also reduces indoor air quality, as buildings which were originally designed for natural ventilation are made highly air tight. A solution to the problem would be a mechanical ventilation system, but several problems are experienced when implementing it - no place for installing air ducts, cold air inflow or additional energy needed for incoming air preheating. Ventilation using heat recovery units is the one method out of many other energy saving measures. The advantage of using heat recovery units is energy saving, and as a result, savings on costs of the operation of the ventilation system. This paper describes the renovation carried out in 4-storey apartment buildings. In the course of the study, two buildings were analysed, both belonging to the series buildings of the Soviet Era (103 series), built in 1970 using the same materials, the same construction solutions. The renovation was carried out by one company, using the same materials and the same renovation principles for both buildings. The only difference after the renovation is that a new centralized mechanical ventilation system with a heat recovery unit is installed in one of the buildings, while in the second building the natural ventilation system is preserved. The arrangement of the mechanical ventilation system is rather innovative as the ventilation ducts in the building for fresh air supply are integrated into the facade's insulation layer and enter the living room through the wall directly behind heating radiators. The main questions studied in the course of the research are the efficiency of the mechanical ventilation system heat recovery, the building's air tightness, and the overall system efficiency.