From which part of a building is the return air not allowed to be taken?

Return air is not allowed to be taken from unconditioned spaces like attics or boiler rooms primarily because these areas do not have controlled environments and may contain contaminants or harmful gases that can compromise air quality. Unconditioned attics can have extreme temperatures, while boiler rooms could have combustible gases, moisture, or other pollutants. Drawing air from these areas can adversely affect the indoor air quality and the efficiency of the HVAC system.

Proper design of HVAC systems emphasizes sourcing return air from living areas or spaces that are conditioned and regularly occupied. This helps in maintaining balanced airflow and ensuring the air circulated throughout the building is clean and safe for occupants. The other areas like kitchens, living rooms, offices, guest rooms, storage closets, and laundry rooms are generally acceptable for return air as they are part of the conditioned living space where air quality can be managed effectively.