1. What is the Duct Size Formula?

The duct size formula (A = CFM / V) calculates the required cross-sectional area of a duct based on the airflow rate (CFM) and desired air velocity (V). This ensures proper airflow distribution in HVAC systems while minimizing pressure drop and noise.

2. How Does the Calculator Work?

The calculator uses the duct sizing 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 appropriate duct size for efficient system operation.

3. Importance of Proper Duct Sizing

Details: Correct duct sizing is essential for maintaining system efficiency, ensuring proper air distribution, minimizing energy consumption, and reducing noise levels in HVAC systems.

4. Using the Calculator

Tips:

• Enter CFM based on your HVAC system requirements
• Typical velocity ranges: 600-900 fpm for main ducts, 500-700 fpm for branch ducts
• Higher velocities may cause excessive noise and pressure drop
• All values must be positive numbers

5. Frequently Asked Questions (FAQ)

Q1: What are typical velocity limits for different duct types?
A: Supply ducts: 600-900 fpm, Return ducts: 500-700 fpm, Residential: 500-800 fpm, Commercial: 1000-2000 fpm.

Q2: How does duct shape affect the calculation?
A: The formula gives area - for rectangular ducts, you'll need to choose dimensions that provide this area while maintaining proper aspect ratio.

Q3: What's the relationship between velocity and pressure drop?
A: Pressure drop increases with the square of velocity - doubling velocity quadruples pressure drop.

Q4: Should I include safety factors in my calculation?
A: It's common to add 10-20% to the calculated area to account for future expansion or system imperfections.

Q5: How does altitude affect duct sizing?
A: At higher altitudes, air is less dense, so CFM values may need adjustment for equivalent mass flow rates.