1. What is the Duct Size Formula?

The duct size formula calculates the required cross-sectional area of HVAC ducts for residential systems based on airflow (CFM) and air velocity. Proper duct sizing is essential for efficient system operation and comfort.

2. How Does the Calculator Work?

The calculator uses the fundamental 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 proper duct dimensions.

3. Importance of Proper Duct Sizing

Details: Correct duct sizing ensures proper airflow distribution, maintains system efficiency, prevents noise issues, and provides balanced comfort throughout the home.

4. Using the Calculator

Tips: Enter CFM based on your HVAC system capacity and select appropriate velocity (typically 600-900 fpm for supply, 500-700 fpm for return in residential systems).

5. Frequently Asked Questions (FAQ)

Q1: What are typical velocity limits for residential ducts?
A: Supply ducts: 600-900 fpm, Return ducts: 500-700 fpm, to balance noise and efficiency.

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

Q3: What's the relationship between duct size and static pressure?
A: Undersized ducts increase static pressure, reducing airflow and efficiency while increasing noise.

Q4: Should I use the same size for all ducts?
A: No, ducts should be sized proportionally to the CFM requirements of each branch using duct sizing charts or manuals.

Q5: How does altitude affect duct sizing?
A: At higher altitudes, air is less dense, requiring larger ducts for equivalent CFM at sea level.