The bending radius of flexible duct should be at least equal to its diameter.
Proper bending radius is crucial for efficient airflow in HVAC systems. When flexible duct is bent too tightly, it restricts airflow, increases static pressure, and reduces system performance.
Based on common standards and practices:
- The center line of a bend in flexible duct should follow a curve with a radius of at least the duct's inside diameter.
For example, as noted in the reference provided:
For example, the center line of a bend in 8-inch–diameter duct should follow a curve with a radius of at least 8 inches [3C]. One easy way to ensure that bends meet this standard is to use metal elbows at these critical locations.
This means if you have 6-inch diameter flexible duct, the bend's center line radius should be at least 6 inches. For 10-inch duct, the radius should be at least 10 inches, and so on.
Why is Bending Radius Important?
- Airflow Efficiency: Maintains smooth airflow, reducing turbulence and pressure drop.
- System Performance: Ensures the HVAC system delivers the intended amount of conditioned air.
- Energy Consumption: Reduces the workload on the fan, potentially lowering energy costs.
- Duct Longevity: Minimizes stress on the duct material.
How to Ensure Proper Bending Radius
- Avoid Sharp Turns: Never bend flexible duct into a sharp corner or fold it.
- Gentle Curves: Aim for smooth, gradual bends.
- Measure: If unsure, use a string or measuring tape to check the radius of the center line of the bend.
- Use Hard Pipe Fittings: As suggested in the reference, using metal elbows (like 90-degree or 45-degree rigid elbows) at critical turns ensures the bend radius is maintained and provides a more robust connection than tight flexible duct bends.
Minimum Bending Radius Examples
Here are some examples based on the general rule:
| Duct Diameter | Minimum Center Line Bending Radius |
|---|---|
| 4 inches | At least 4 inches |
| 6 inches | At least 6 inches |
| 8 inches | At least 8 inches |
| 10 inches | At least 10 inches |
| 12 inches | At least 12 inches |
Adhering to this minimum radius is a key step in proper flexible duct installation.
