1. What is the Duct Size Formula?

The duct size formula \( A = \frac{CFM}{V} \) calculates the cross-sectional area (A) of a duct needed to handle a specific airflow (CFM) at a given velocity (V). This is fundamental for HVAC system design to ensure proper airflow while minimizing noise and pressure drop.

2. How Does the Calculator Work?

The calculator uses the basic 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.

3. Importance of Proper Duct Sizing

Details: Correct duct sizing is crucial for efficient HVAC operation. Undersized ducts increase noise and static pressure, while oversized ducts waste materials and may lead to poor air distribution.

4. Using the Calculator

Tips:

• Typical velocity ranges: 600-900 fpm for supply, 500-700 fpm for return
• Higher velocities may be used in main ducts (up to 1200 fpm)
• Lower velocities (300-500 fpm) for quiet applications

5. Frequently Asked Questions (FAQ)

Q1: What's the typical velocity for residential systems?
A: 600-900 fpm for supply ducts, 500-700 fpm for return ducts.

Q2: How do I convert square feet to round duct diameter?
A: Diameter = 2 × √(Area/π). For rectangular ducts, common aspect ratios are 1:1 to 4:1.

Q3: What factors affect duct sizing?
A: Airflow requirements, velocity limits, duct material, system static pressure, and noise considerations.

Q4: Should I use higher velocities to save space?
A: Higher velocities reduce duct size but increase noise and pressure drop. Balance is key.

Q5: Are there standard duct sizes?
A: Yes, ducts typically come in standard sizes (4", 6", 8", etc.), so round up to nearest standard size.