In modern ventilation systems, heat recovery transfers a large proportion of the thermal energy from exhaust air to fresh outdoor air. Common systems include cross-counterflow plate heat exchangers, rotary heat exchangers (enthalpy wheels), and heat pump circuits. The thermal efficiency, expressed as the heat recovery coefficient, can exceed 90% in high-quality units. Enthalpy exchangers additionally transfer moisture, which is especially beneficial during dry winter months. Testing of heat exchangers is governed by EN 308. Proper design prevents icing, short-circuit airflow, and hygiene issues. Heat recovery is a central element of energy-efficient buildings and a prerequisite for low-energy and passive house standards.