1. What is the Duct Size Equation?

The duct size equation calculates the required cross-sectional area of an HVAC duct based on the airflow rate (CFM) and desired air velocity. Proper duct sizing is essential for efficient HVAC system operation.

2. How Does the Calculator Work?

The calculator uses the duct area equation:

Where:

• \( A \) — Duct cross-sectional area (square feet)
• \( CFM \) — Airflow rate (cubic feet per minute)
• \( V \) — Air velocity (feet per minute)

Explanation: The equation balances airflow requirements with velocity constraints to determine the minimum duct area needed.

3. Importance of Proper Duct Sizing

Details: Correct duct sizing ensures proper airflow, minimizes energy loss, reduces noise, and maintains system efficiency. Undersized ducts increase static pressure while oversized ducts waste materials and space.

4. Using the Calculator

Tips: Enter CFM (typically 400-2000 for residential systems) and desired velocity (typically 600-900 fpm for supply, 500-700 fpm for return). All values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: What's the typical air velocity for residential systems?
A: Supply ducts: 600-900 fpm, Return ducts: 500-700 fpm, depending on noise requirements.

Q2: How does duct shape affect sizing?
A: Round ducts are more efficient than rectangular. Equivalent round diameter can be calculated from area.

Q3: What about duct friction loss?
A: This calculator provides basic sizing. For complete design, friction rate calculations are also needed.

Q4: Can I use this for commercial systems?
A: Yes, but commercial systems often use higher velocities (up to 2000 fpm in mains).

Q5: How do I convert to round duct diameter?
A: Diameter = 2 × √(Area/π) for equivalent round duct size.