1. What is the Duct Sizing Formula?

The duct sizing formula calculates the cross-sectional area needed for air ducts based on 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 formula:

Where:

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

Explanation: The formula determines the required duct size to maintain proper air velocity for a given airflow rate. Higher velocities create more noise but allow smaller ducts, while lower velocities require larger ducts but operate more quietly.

3. Importance of Proper Duct Sizing

Details: Correct duct sizing ensures efficient airflow, proper system balance, adequate air delivery to all spaces, and optimal energy efficiency. Undersized ducts increase static pressure and reduce airflow, while oversized ducts can lead to poor air distribution and higher installation costs.

4. Using the Calculator

Tips:

• Typical velocity ranges: 600-900 fpm for main ducts, 500-700 fpm for branch ducts
• Residential systems typically use 400-600 fpm for quiet operation
• Commercial systems may use higher velocities (up to 2000 fpm for high-pressure systems)
• Enter CFM based on your HVAC system requirements

5. Frequently Asked Questions (FAQ)

Q1: What's the recommended velocity for residential systems?
A: For quiet operation, 400-600 fpm is recommended in living spaces. Higher velocities may be acceptable in less sensitive areas.

Q2: How do I convert duct area to round duct diameter?
A: For round ducts: Diameter = 2 × √(Area/π). The calculator provides area in both sq ft and sq in for convenience.

Q3: What about rectangular ducts?
A: Rectangular ducts should maintain similar cross-sectional area. Common aspect ratios are 1:1 (square) to 4:1 (flat rectangle).

Q4: Does duct material affect sizing?
A: Material affects friction loss but not the basic area calculation. Different materials may require adjustments for equivalent friction.

Q5: How does altitude affect duct sizing?
A: At higher altitudes, air is less dense, so CFM requirements may change but velocity recommendations remain similar.