Understanding HVAC Duct System Design
This calculator designs an HVAC duct system for multiple rooms by calculating duct sizes based on room size (CFM estimated at 1 CFM per sq. ft.) or manual CFM inputs. It accounts for velocity, duct shape, friction loss, and fittings to ensure efficient airflow and minimal pressure loss.
Proper duct design prevents issues like high static pressure, noise, or uneven cooling/heating. The equal friction method balances duct sizes for consistent performance. Typical supply duct velocities are 800–1000 FPM, and friction rates for smooth metal ducts are around 0.1 in. w.g./100 ft.
Example Duct Design Results
| Parameter | Room 1 | Room 2 | Room 3 | Scenario |
|---|---|---|---|---|
| Airflow (CFM) | 200 | 300 | 400 | Based on 200, 300, 400 sq. ft. |
| Duct Area (sq. in.) | 28.8 | 43.2 | 57.6 | 1000 FPM |
| Round Diameter (in.) | 6.1 | 7.4 | 8.6 | Standard sizes: 6, 8, 8 in. |
| Rectangular Dimensions (in.) | 7 x 4 | 9 x 5 | 10 x 6 | 2:1 aspect ratio |
| Pressure Drop (in. w.g.) | 0.13 | 0.13 | 0.13 | 100 ft, 0.1 in. w.g./100 ft, 2 90° elbows |
How to Use the Air Duct Design Calculator
- Enter the number of rooms in the HVAC system (default: 3).
- For each room, input square footage (to estimate CFM) or manual CFM, duct length, and number/type of elbows.
- Specify the desired velocity (FPM, typically 800–1000), duct shape, aspect ratio (for rectangular ducts), and friction rate.
- Click "Calculate Duct Design" to see duct sizes, pressure drops, and total system pressure drop.
Duct Design Comparison
Duct area and pressure drop across rooms for 1000 FPM, 0.1 in. w.g./100 ft.
Frequently Asked Questions
How is CFM estimated for a room?
CFM is estimated at 1 CFM per square foot for residential cooling, or you can input manual CFM based on HVAC unit specs or load calculations.
What is the equal friction method?
The equal friction method sizes ducts to maintain a constant friction rate (e.g., 0.1 in. w.g./100 ft), balancing airflow and pressure for efficient HVAC performance.