What is the purpose of a relief fan in HVAC systems?

• A. To provide flexibility in location and reduce energy use compared to a return fan.
• B. To humidify the air by introducing moisture.
• C. To filter contaminants from supply air.
• D. To cool equipment in the mechanical room.

Answer: (C)

Relief fans serve to remove air from a space to the outdoors to balance the air that is being supplied and to prevent over-pressurization. In ventilation systems, especially when outdoor air is introduced or when there’s significant exhaust, the space can become too positively pressurized or too pressure-laden. The relief path gives a controlled way for that air to escape, helping maintain the intended room pressure and stable ventilation rates. This keeps occupants comfortable and prevents air from leaking through doors and cracks. It’s not about adding moisture, filtering, or cooling; those tasks are handled by humidifiers, filters, and other mechanical components. So the relief fan’s main role is balancing and exhausting excess air to maintain proper pressure and ventilation performance.