ACCA Manual D • 0.08 in. wg Friction1600 CFM • 4 TonsWhisper Quiet (<700 FPM)
1,600 CFM (4 Ton) Return Air Duct & Filter Grille Size
Calculated duct sizing specifications for 1600 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 Size18" RoundSpiral / Snaplock Pipe
Air Velocity905 FPMFeet Per Minute
Cooling Tonnage4 Tons@ 400 CFM / Ton
Register / GrilleDualFace Dimension
Equivalent Rectangular Duct Sizing Schedule
Exact rectangular sheet metal dimensions offering equivalent airflow resistance to a 18" round pipe
| Height (Depth) | Width | Cross Area | Air Velocity | Aspect Ratio | SMACNA Status |
|---|---|---|---|---|---|
| 12" Deep | 32" Wide | 384 sq in | 600 FPM | 2.7:1 | Optimal (<2:1) |
| 14" Deep | 28" Wide | 392 sq in | 588 FPM | 2.0:1 | Optimal (<2:1) |
| 16" Deep | 24" Wide | 384 sq in | 600 FPM | 1.5:1 | Optimal (<2:1) |
Air Terminal & Grille Sizing Guidelines
Recommended face velocities to prevent whistling diffuser vanes
Supply Register Specifications
Recommended Grille Size:Dual 20x20 or Dual 16x25 Return Filter Grilles
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
- 4-ton systems should have return air pulled from both upstairs and downstairs to balance home pressure.
- A 14x28 or 16x24 return drop prevents starved blower motors and frozen evaporator coils.
Frequently Asked Duct Sizing Questions
Can a 4-ton AC use a single return air grille?
Only if the single return grille is massive (at least 24x30 inches). HVAC engineers strongly recommend two separate return grilles to distribute air intake evenly.