HVAC Duct Sizer + Superheat/Subcool
Reference helper for quick planning. Velocity sizing is shown here; verify friction-rate and code/manufacturer limits per your method.
| Target | Area | Round ∅ | Rect ∅ |
|---|---|---|---|
| Min velocity | 0.00 in² | - | - |
| Mid velocity | 0.00 in² | - | - |
| Max velocity | 0.00 in² | - | - |
Sizing uses area = flow / velocity. This tool does not compute friction loss; confirm against your friction-rate method.
If a fixed width is provided, height is solved from area; otherwise width/height follow the aspect ratio.
HVAC Duct Design Guide
Sizing Methods: Contextualizing the Calculation
This tool primarily uses the Velocity Reduction Method logic (Area = Flow / Velocity) to give you a starting size. However, professional design often involves:
- Equal Friction Method: The most common for residential (ACCA Manual D). It assumes a constant pressure drop (e.g., 0.1" w.c./100 ft). It is "self-balancing" to a degree but requires dampers.
- Velocity Reduction: Used for main trunks to control noise.
- Static Regain: Complex method for high-velocity commercial systems.
The Friction Rate Debate
While velocity is a good check for noise, friction rate is critical for fan performance. Standard design values often used:
- Supply Ducts: 0.10 in. w.c. per 100 ft (Standard Residential)
- Return Ducts: 0.08 in. w.c. per 100 ft (To reduce noise)
- Commercial: 0.10 - 0.15 in. w.c. per 100 ft
Shape Efficiency & Equivalent Diameter
Rectangular ducts are less efficient than round ducts. To move the same air with the same friction, a rectangular duct must be slightly larger than its round area equivalent. This tool calculates the Equivalent Diameter using the Huebscher Equation:
$$D_{eq} = \frac{1.30 \times (a \times b)^{0.625}}{(a + b)^{0.25}}$$
Where a and b are the sides of the rectangular duct.