ACCA Manual D • 0.1 in. wg Friction1800 CFM • 4.5 TonsStandard Residential (<900 FPM)
1,800 CFM Duct Size: 4.5 Ton HVAC Main Trunk Line
Calculated duct sizing specifications for 1800 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 Velocity1019 FPMFeet Per Minute
Cooling Tonnage4.5 Tons@ 400 CFM / Ton
Register / Grille14x28Face 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 | 30" Wide | 360 sq in | 720 FPM | 2.5:1 | Optimal (<2:1) |
| 14" Deep | 26" Wide | 364 sq in | 712 FPM | 1.9:1 | Optimal (<2:1) |
Air Terminal & Grille Sizing Guidelines
Recommended face velocities to prevent whistling diffuser vanes
Supply Register Specifications
Recommended Grille Size:14x28 Supply Plaque / Dual 20x20 Return 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
- Use 18-inch round metal or 14x26 rectangular duct to maintain smooth air transition.
- Support heavy duct runs with 1-inch galvanized hanger straps spaced every 8 feet per SMACNA guidelines.
Frequently Asked Duct Sizing Questions
What size duct moves 1,800 CFM?
1,800 CFM requires an 18-inch round duct or a 14x26 rectangular supply trunk.