ACCA Manual D • 0.08 in. wg Friction350 CFM • 0.875 TonsWhisper Quiet (<700 FPM)
350 CFM Duct Size: Bonus Room & Finished Attic Branch
Calculated duct sizing specifications for 350 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 Size9" RoundSpiral / Snaplock Pipe
Air Velocity792 FPMFeet Per Minute
Cooling Tonnage0.875 Tons@ 400 CFM / Ton
Register / Grille8x14Face Dimension
Equivalent Rectangular Duct Sizing Schedule
Exact rectangular sheet metal dimensions offering equivalent airflow resistance to a 9" round pipe
| Height (Depth) | Width | Cross Area | Air Velocity | Aspect Ratio | SMACNA Status |
|---|---|---|---|---|---|
| 8" Deep | 14" Wide | 112 sq in | 450 FPM | 1.8:1 | Optimal (<2:1) |
| 6" Deep | 18" Wide | 108 sq in | 467 FPM | 3.0:1 | Acceptable (<3:1) |
Air Terminal & Grille Sizing Guidelines
Recommended face velocities to prevent whistling diffuser vanes
Supply Register Specifications
Recommended Grille Size:8x14 or Dual 6x12 Registers
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 9-inch round duct (or 10-inch flex duct) provides excellent airflow with low static pressure resistance.
- Insulate attic ductwork to minimum R-8 with sealed mastic joints per IECC requirements.
Frequently Asked Duct Sizing Questions
What size duct is required for 350 CFM?
A 9-inch round metal duct or an 8x14 rectangular trunk is optimal for 350 CFM.