ACCA Manual D • 0.1 in. wg Friction800 CFM • 2 TonsStandard Residential (<900 FPM)
800 CFM Duct Size: 2.0 Ton HVAC Main Trunk Line
Calculated duct sizing specifications for 800 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 Size12" RoundSpiral / Snaplock Pipe
Air Velocity1019 FPMFeet Per Minute
Cooling Tonnage2 Tons@ 400 CFM / Ton
Register / Grille10x20Face Dimension
Equivalent Rectangular Duct Sizing Schedule
Exact rectangular sheet metal dimensions offering equivalent airflow resistance to a 12" round pipe
| Height (Depth) | Width | Cross Area | Air Velocity | Aspect Ratio | SMACNA Status |
|---|---|---|---|---|---|
| 8" Deep | 22" Wide | 176 sq in | 655 FPM | 2.8:1 | Acceptable (<3:1) |
| 10" Deep | 18" Wide | 180 sq in | 640 FPM | 1.8:1 | Optimal (<2:1) |
| 12" Deep | 16" Wide | 192 sq in | 600 FPM | 1.3:1 | Optimal (<2:1) |
Air Terminal & Grille Sizing Guidelines
Recommended face velocities to prevent whistling diffuser vanes
Supply Register Specifications
Recommended Grille Size:10x20 Supply Plaque / 16x20 Return 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
- While a 12-inch round duct handles 800 CFM, velocity is 1,019 FPM. For basement main trunks, a 10x18 rectangular duct reduces velocity to 640 FPM for quieter air delivery.
- Always transition from the furnace plenum using an aerodynamic turning vane elbow.
Frequently Asked Duct Sizing Questions
What size duct does a 2-ton AC need?
A 2-ton AC system requires 800 CFM of airflow, which calls for a 12-inch to 14-inch round duct or a 10x18 rectangular main trunk.