ACCA Manual D • 0.1 in. wg Friction1400 CFM • 3.5 TonsStandard Residential (<900 FPM)
1,400 CFM Duct Size: 3.5 Ton HVAC Main Trunk Line
Calculated duct sizing specifications for 1400 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 Velocity1003 FPMFeet Per Minute
Cooling Tonnage3.5 Tons@ 400 CFM / Ton
Register / Grille12x24Face 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 | 720 FPM | 2.8:1 | Optimal (<2:1) |
| 12" Deep | 24" Wide | 288 sq in | 700 FPM | 2.0:1 | Optimal (<2:1) |
Air Terminal & Grille Sizing Guidelines
Recommended face velocities to prevent whistling diffuser vanes
Supply Register Specifications
Recommended Grille Size:12x24 Supply Plaque / 20x25 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 16-inch round duct or 12x24 rectangular trunk provides ideal static pressure delivery for 3.5 ton heat pumps and AC units.
- Reduce trunk dimensions proportionally as branch run takeoffs diverge to keep static pressure balanced down the run.
Frequently Asked Duct Sizing Questions
What duct size is needed for 3.5 tons?
A 3.5-ton system delivering 1,400 CFM requires a 16-inch round pipe or a 12x24 rectangular main trunk.