What is the main restriction regarding return air from one residential living unit?

The main restriction regarding return air from one residential living unit is that it shall not be mixed with other living units. This requirement is in place to ensure that the indoor air quality and comfort of each living unit is maintained and that the systems operate efficiently. Allowing return air to mix between units could lead to cross-contamination of air, where pollutants, odors, and other elements from one unit could negatively impact another. This restriction also helps to comply with codes that prioritize tenant privacy and health, as different living units may have different air quality standards and needs. This separation ensures that each unit's HVAC system is serving only its intended space, thus promoting better air quality and reducing the risk of spreading allergens or airborne illnesses between units.