1. What is the Duct Size Formula?

The duct size formula calculates the required cross-sectional area of a duct based on airflow (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 basic duct sizing equation:

Where:

• \( A \) — Duct cross-sectional area (square feet)
• \( CFM \) — Airflow in cubic feet per minute
• \( V \) — Air velocity in 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 ensures proper air distribution, minimizes energy loss, reduces noise, and maintains system efficiency.

4. Using the Calculator

Tips: Enter CFM (airflow) and desired velocity (typically 600-900 fpm for supply, 500-700 fpm for return). The calculator provides both area and equivalent round duct diameter.

5. Frequently Asked Questions (FAQ)

Q1: What are typical air velocities for HVAC ducts?
A: Supply ducts: 600-900 fpm, Return ducts: 500-700 fpm, Low-velocity systems: 300-500 fpm.

Q2: How does duct shape affect sizing?
A: Round ducts are more efficient but rectangular ducts are often used for space constraints. The calculator provides equivalent round diameter.

Q3: What about friction loss?
A: This basic formula doesn't account for friction. For long duct runs, additional calculations are needed to ensure proper static pressure.

Q4: How does altitude affect duct sizing?
A: At higher altitudes, air density decreases, requiring larger ducts for the same mass flow rate.

Q5: What's the maximum recommended velocity?
A: Generally 1200 fpm for main ducts, 700-900 fpm for branch ducts to avoid excessive noise and pressure drop.