To choose the right size of rectangular flexible duct, follow these steps:

1. Determine Airflow Needs: Calculate the required airflow volume (CFM) for your HVAC system based on heating and cooling loads.

2. Check Duct Velocity: Ensure that the air velocity in the duct stays within recommended ranges to avoid excessive noise and pressure loss.

3. Consider System Pressure: Account for the static pressure of your HVAC system to select a duct that can handle the pressure without compromising performance.

4. Dimensional Fit: Measure the space where the duct will be installed to ensure the selected size will fit properly without interference.

5. Allow for Insulation: If using insulated duct, consider the added thickness when determining the physical dimensions needed for installation.

6. Consult Manufacturer Guides: Use manufacturers’ sizing charts or software tools to select the appropriate duct size based on your specific requirements.

7. Seek Professional Advice: If unsure, consult with an HVAC professional who can help you determine the optimal duct size for your system.

By following these steps, you can accurately size a rectangular flexible duct to meet the performance needs of your HVAC system while also fitting seamlessly into your specified space.