What is the role of a return fan with outside air injection in HVAC systems?

• A. To control building pressure and maintain minimum outside air.
• B. To heat return air to a higher temperature.
• C. To increase ventilation rates.
• D. To filter outside air.

Answer: (A)

A return fan with outside air injection is used to manage the building’s pressure relationship with the outdoors while guaranteeing a minimum amount of outdoor air for ventilation. The fan helps pull air from the space back to the air handling unit, and by injecting a controlled amount of outdoor air into that return stream, the system sets and maintains the desired pressure in the building (preventing unwanted infiltration or exfiltration) and ensures a baseline outdoor air portion is introduced for ventilation. Heating or filtering functions are handled by other components in the system, so the primary role here is balancing pressure and ensuring minimum outdoor air.