ACCA Manual D • 0.08 in. wg Friction1200 CFM • 3 TonsWhisper Quiet (<700 FPM)
1,200 CFM (3 Ton) Return Air Duct & Filter Grille Size
Calculated duct sizing specifications for 1200 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 Size16" RoundSpiral / Snaplock Pipe
Air Velocity859 FPMFeet Per Minute
Cooling Tonnage3 Tons@ 400 CFM / Ton
Register / Grille20x25Face Dimension
Equivalent Rectangular Duct Sizing Schedule
Exact rectangular sheet metal dimensions offering equivalent airflow resistance to a 16" round pipe
| Height (Depth) | Width | Cross Area | Air Velocity | Aspect Ratio | SMACNA Status |
|---|---|---|---|---|---|
| 10" Deep | 28" Wide | 280 sq in | 617 FPM | 2.8:1 | Optimal (<2:1) |
| 12" Deep | 24" Wide | 288 sq in | 600 FPM | 2.0:1 | Optimal (<2:1) |
| 14" Deep | 20" Wide | 280 sq in | 617 FPM | 1.4:1 | Optimal (<2:1) |
Air Terminal & Grille Sizing Guidelines
Recommended face velocities to prevent whistling diffuser vanes
Supply Register Specifications
Recommended Grille Size:20x25 or 20x30 Filter Grille (Minimum 500-600 sq in face area)
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 3-ton return must move 1,200 CFM. Never choke a 3-ton unit with a single 16x20 return filter.
- Use a 12x24 return drop and at least a 20x25 4-inch deep pleated media air cleaner.
Frequently Asked Duct Sizing Questions
What size return duct is needed for a 3-ton AC?
A 3-ton AC requires a 16-inch round duct or a 12x24 rectangular return drop, accompanied by a 20x25 or larger filter.