Quickly estimate the proper round or rectangular duct dimensions needed to maintain optimal air velocity for a given airflow (CFM).
Typical: 400 CFM per Ton of cooling.
Typical main trunk: 700-900 FPM.
*This is a simplified velocity-based sizing calculation. A complete Manual D calculation considering friction rate and equivalent lengths is required for actual system design.
Undersized ductwork restricts airflow, causing the system to work harder. This leads to loud air noise, uneven room temperatures, frozen evaporator coils, and premature blower motor failure.
Ductwork is sized based on the CFM (Cubic Feet per Minute) of air the HVAC system produces, balancing the air velocity and static pressure to ensure quiet and efficient airflow to every room.
Static pressure is the resistance to airflow within the duct system. High static pressure—often caused by small ducts or dirty filters—reduces system efficiency and strains the equipment.
