ACCA Manual D • 0.08 in. wg Friction750 CFM • 1.875 TonsWhisper Quiet (<700 FPM)
14-Inch Flex Duct CFM Capacity & Sizing Guide
Calculated duct sizing specifications for 750 CFM of conditioned airflow. Sized via the Equal Friction Method and Huebscher equivalent rectangular equation to maintain static pressure and eliminate air rush noise.
Round Duct Size14" RoundSpiral / Snaplock Pipe
Air Velocity701 FPMFeet Per Minute
Cooling Tonnage1.875 Tons@ 400 CFM / Ton
Register / Grille16x25Face Dimension
Equivalent Rectangular Duct Sizing Schedule
Exact rectangular sheet metal dimensions offering equivalent airflow resistance to a 14" round pipe
| Height (Depth) | Width | Cross Area | Air Velocity | Aspect Ratio | SMACNA Status |
|---|---|---|---|---|---|
| 10" Deep | 22" Wide | 220 sq in | 491 FPM | 2.2:1 | Optimal (<2:1) |
Air Terminal & Grille Sizing Guidelines
Recommended face velocities to prevent whistling diffuser vanes
Supply Register Specifications
Recommended Grille Size:16x25 Return Filter Grille
Maximum Target Velocity:500–700 FPM at register face
Diffuser Throw:Spread toward outside walls
Return Air Intake Rules
Return Velocity Target:<450 FPM through filter face
Filter Surface Requirement:Minimum 2 sq ft per Ton
Return Friction Target:0.05 – 0.08 in. wg / 100 ft
HVAC Sheet Metal & Installation Directives
- A 14-inch flex duct delivers 700 to 800 CFM, making it the premier choice for 2-ton return air ducts in attic-mounted air handler systems.
- Ensure the vinyl outer vapor barrier is completely sealed with UL 181-FX foil tape to prevent attic humidity intrusion.
Frequently Asked Duct Sizing Questions
How much air will a 14-inch flex duct move?
A 14-inch flex duct moves roughly 750 CFM at 0.08 in. wg and up to 880 CFM at 0.10 in. wg.