This paper examines decentralized supply-and-exhaust ventilation systems (DSEVS) with heat recovery as an energy-efficient solution for airtight buildings, where natural ventilation becomes ineffective due to modern energy-saving standards and climatic variations. The study evaluates the performance (COP) of three systems equipped with copper heat exchangers, compares them with centralized systems, and identifies challenges related to airflow balancing under real-world conditions. Experimental results revealed a laboratory COP of 85–91%, which dropped to 51–59% in real buildings due to airflow imbalance (up to 64%) and infiltration. The findings indicate that efficiency strongly depends on airtightness and pressure stability, exposing design flaws in systems with significant imbalances. Decentralized systems offer advantages in ease of installation and adaptability, but require further refinement. This research emphasizes the need to improve airtightness and airflow balance to enhance COP. A particular focus is directed towards hybrid systems integrating DSEVS with natural ventilation, proposed as a promising direction for future research. These solutions hold potential for stable air exchange and heat retention tailored to the climatic conditions of Ukraine and Europe. We recommend the development of counterflow hybrid DSEVS as a strategy for upgrading ventilation in modern buildings.