Understanding HVAC Duct Sizing
Proper duct sizing ensures efficient airflow, minimizes energy waste, and reduces noise in HVAC systems. This calculator determines duct dimensions based on airflow (CFM), desired velocity (FPM), and duct shape, while accounting for friction loss and fittings like elbows.
Undersized ducts increase static pressure and noise, while oversized ducts waste space and energy. The equal friction method balances airflow and pressure for optimal performance. Typical supply duct velocities are 800–1000 FPM, and friction rates are around 0.1 in. w.g./100 ft for smooth metal ducts.
Example Duct Sizing Results
| Parameter | Value | Scenario |
|---|---|---|
| Duct Area | 36 sq. in. | 400 CFM, 1600 FPM |
| Round Diameter | 6.8 in. | Standard 6 in. duct |
| Rectangular Dimensions | 8 x 4.5 in. | 2:1 aspect ratio |
| Pressure Drop | 0.15 in. w.g. | 100 ft, 0.1 in. w.g./100 ft, 2 90° elbows |
| Velocity | 1600 FPM | Calculated from inputs |
How to Use the Air Duct Sizing Calculator
- Enter the airflow (CFM) required for the space, typically 400 CFM per ton of HVAC capacity.
- Specify the desired velocity (FPM), usually 800–1000 FPM for supply ducts to minimize noise.
- Select duct shape (round or rectangular) and, for rectangular ducts, choose an aspect ratio.
- Enter friction rate, duct length, and number/type of elbows for pressure drop calculations.
- Click "Calculate Duct Size" to see the duct area, dimensions, velocity, and pressure drop.
Duct Sizing Comparison
Area and velocity for 400 CFM at 1600 FPM, round vs. rectangular ducts.
Frequently Asked Questions
Why is proper duct sizing important?
Correct duct sizing ensures efficient airflow, reduces energy consumption, minimizes noise, and extends HVAC equipment lifespan.
What is the ideal velocity for HVAC ducts?
Supply ducts typically aim for 800–1000 FPM to balance efficiency and noise. Return ducts may use lower velocities (600–800 FPM).