The paper presents the results of the investigations into the indoor climate and energy efficiency of ventilation devices in residential buildings and nursing homes. The indoor climate studies discussed in the article were conducted in a nursing home where room ventilation is based on Meltem Air Handling Units (AHU). Similar studies have been carried out in apartment buildings where CO₂ levels in bedrooms were compared before and after renovating the ventilation with Meltem AHU and installing exhaust fans in the bathroom/WC and kitchen. Tenants evaluate the use of Meltem AHU in apartments very positively. The article presents the efficiency results of Meltem AHU with different external air temperatures conducted in apartments in real-life situations. The study shows that skilful renovation of ventilation in old apartment buildings enables to achieve good indoor climate and energy saving at the same time.