Return air from one dwelling unit should not be discharged into which of the following?

When considering the proper ventilation practices as dictated by the mechanical code, it is essential to understand why return air from one dwelling unit should not be discharged into another dwelling unit. The primary concern is the potential for air quality issues and the transfer of contaminants. Each dwelling unit is typically designed to maintain its own indoor air quality, insulation, and environmental control, often separate from others.

Discharging return air from one unit into another could facilitate the movement of odors, airborne pathogens, and other pollutants, which may arise from cooking, cleaning supplies, or even activities like smoking. This can degrade the living environment and comfort of the occupants in the receiving unit.

Additionally, such a practice could violate building codes that are designed to ensure that each dwelling unit sustains its own clean air supply and retains control over its specific internal conditions. Thus, ensuring that return air from one dwelling unit does not enter another maintains the integrity of each unit's air quality, safety, and comfort.

The other options, while also important factors in the overall design of ventilation systems, do not present the same level of concern regarding direct air quality exchange between independent living spaces. For example, common areas may have specific ventilation strategies that include return air provisions designed for those shared spaces.