ACCA Manual D • 0.08 in. wg Friction2000 CFM • 5 TonsWhisper Quiet (<700 FPM)
2,000 CFM (5 Ton) Return Air Duct & Dual Drop Sizing
Calculated duct sizing specifications for 2000 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 Size20" RoundSpiral / Snaplock Pipe
Air Velocity917 FPMFeet Per Minute
Cooling Tonnage5 Tons@ 400 CFM / Ton
Register / GrilleDualFace Dimension
Equivalent Rectangular Duct Sizing Schedule
Exact rectangular sheet metal dimensions offering equivalent airflow resistance to a 20" round pipe
| Height (Depth) | Width | Cross Area | Air Velocity | Aspect Ratio | SMACNA Status |
|---|---|---|---|---|---|
| 14" Deep | 34" Wide | 476 sq in | 605 FPM | 2.4:1 | Optimal (<2:1) |
| 16" Deep | 30" Wide | 480 sq in | 600 FPM | 1.9:1 | Optimal (<2:1) |
| 18" Deep | 28" Wide | 504 sq in | 571 FPM | 1.6:1 | Optimal (<2:1) |
Air Terminal & Grille Sizing Guidelines
Recommended face velocities to prevent whistling diffuser vanes
Supply Register Specifications
Recommended Grille Size:Dual 20x25 Filter Grilles (Minimum 1,000 sq in combined 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
- MANDATORY CONTRACTOR RULE: Never feed a 5-ton furnace blower through a single side knockout. Use dual side return drops or bottom return with a raised base box.
- Each side drop should measure at least 10x24 (1,000 CFM each) entering both left and right cabinet sides.
Frequently Asked Duct Sizing Questions
Why do 5-ton systems need dual return air drops?
A standard furnace side knockout is only 16x25 inches, which restricts airflow above 1,400 CFM. 2,000 CFM through a single side causes extreme noise and blower motor overheating.