Number of air changes/hour in an area is =Total Room Airflow in CFM x 60 / Total Volume of room in cubic feet ACH = CFMx60/Room Volume Example: Say the volume of the room is 1,200 cubic feet, and the volume of air the HVAC system is going to supply is 200 CFM. Thus, ACH = 200×60/1,200 = 10. This ...