To size flexible ductwork, follow these key steps:

1. Determine the necessary measurements, which include:

   – The duct run’s length.

   – The diameter of the duct.

   – The cubic feet per minute (CFM) measure of airflow rate.

2. Use the formula to calculate the required cross-sectional area of the duct:

   A=Q/V*P

   Where:

   – A is the cross-sectional area of the duct (inches).

   – Q is the airflow rate (CFM).

   – V is the velocity of the air (feet per second).

   – P stands for pressure drop (water column in inches).

3. Ensure the air velocity is between 2,000 to 3,000 feet per minute and the pressure drop does not exceed 0.5 inches of water column per 100 feet of duct.

4. Choose a flexible duct with a larger cross-sectional area than the calculated value to prevent restrictions and maintain the desired airflow rate.

Additionally, consider the static pressure of the system, the number and type of registers, and any bends or turns in the ductwork. Manufacturers provide sizing guides; consult them and possibly an HVAC professional for optimal duct diameter and capacity to achieve efficient performance.