ACCA Manual D • 0.08 in. wg Friction150 CFM • 0.375 TonsStandard Residential (<900 FPM)
150 CFM Duct Size: Master Bedroom & Large Suite Branch
Calculated duct sizing specifications for 150 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 Size6" RoundSpiral / Snaplock Pipe
Air Velocity764 FPMFeet Per Minute
Cooling Tonnage0.375 Tons@ 400 CFM / Ton
Register / Grille4x12Face Dimension
Equivalent Rectangular Duct Sizing Schedule
Exact rectangular sheet metal dimensions offering equivalent airflow resistance to a 6" round pipe
| Height (Depth) | Width | Cross Area | Air Velocity | Aspect Ratio | SMACNA Status |
|---|---|---|---|---|---|
| 6" Deep | 8" Wide | 48 sq in | 450 FPM | 1.3:1 | Optimal (<2:1) |
| 4" Deep | 12" Wide | 48 sq in | 450 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:4x12 or 6x10 Supply Register
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 6-inch rigid metal duct handles 150 CFM at 764 FPM. For flex duct, upsize to 7-inch to avoid velocity whistle.
- Use a 4x12 register boot with a curved-blade diffuser to ensure throw covers the outside wall and windows.
Frequently Asked Duct Sizing Questions
Can a 6-inch flex duct handle 150 CFM?
A 6-inch flex duct will be noisy and cause static pressure restriction at 150 CFM (velocity over 850 FPM). A 7-inch or 8-inch flex duct is recommended for 150 CFM.