Net Free Air Volume Calculator

How to Use This Calculator

1. Enter the Airflow Rate (CFM) in cubic feet per minute. This is the total volume of air moving through the system per minute.
2. Enter the Air Changes per Hour (AH). This represents how many times the air in a space is completely replaced per hour.
3. Click the "Calculate" button to compute the Net Free Air Volume.
4. The result will be displayed in cubic feet, showing the actual volume of fresh air entering the space.

Principle and Applications

Net Free Air Volume (NFAV) is a critical parameter in HVAC (Heating, Ventilation, and Air Conditioning) system design. It represents the portion of the total airflow that is actually fresh air from outside, as opposed to recirculated air.

This calculation helps engineers and designers ensure proper ventilation while minimizing energy waste. By knowing the NFAV, they can properly size air handlers, select appropriate filters, and design systems that maintain good indoor air quality without overcooling or overheating.

Example Scenario

Suppose a room has an air handler providing 500 CFM of airflow, and the room requires 4 air changes per hour. Using this calculator:

• CFM = 500
• AH = 4
• NFAV = 500 × (1 - 500/4) = 500 × (1 - 125) = 500 × (-124) = -62,500 cubic feet

In this case, the negative result indicates that the airflow rate is too high relative to the room size, and recirculation would be excessive. The AH value might need to be adjusted or the CFM reduced to achieve a positive NFAV.