1. What is the AC Duct Size Calculation?

The duct size calculation determines the proper cross-sectional area needed for air ducts in HVAC systems based on airflow rate and desired air velocity. Proper sizing is essential for efficient system operation.

2. How Does the Calculator Work?

The calculator uses the fundamental 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 minimum duct area needed.

3. Importance of Proper Duct Sizing

Details: Correct duct sizing ensures efficient airflow, minimizes energy loss, reduces noise, and maintains proper system pressure. Undersized ducts increase static pressure and reduce airflow, while oversized ducts waste materials and space.

4. Using the Calculator

Tips:

• Typical residential air velocity: 600-900 fpm for supply, 500-700 fpm for return
• Commercial systems often use higher velocities (1000-2000 fpm)
• Enter CFM based on your system's airflow requirements
• Select velocity based on application and noise considerations

5. Frequently Asked Questions (FAQ)

Q1: What are typical duct sizes for residential systems?
A: Common residential duct sizes range from 4" to 12" round diameter or equivalent rectangular sizes, depending on the CFM requirements.

Q2: How does duct shape affect sizing?
A: Round ducts are more efficient (less friction) than rectangular ducts. When using rectangular ducts, maintain similar cross-sectional area and aspect ratio (width:height) between 1:1 and 4:1.

Q3: What velocity should I use for quiet systems?
A: For quieter operation, use lower velocities (500-600 fpm for supply, 400-500 fpm for return). Higher velocities increase noise.

Q4: How do I convert area to round duct diameter?
A: Diameter = 2 × √(Area/π). For rectangular ducts, choose dimensions that maintain the calculated area while fitting your space.

Q5: Are there standard duct size charts?
A: Yes, standard duct sizing charts (like the one below) provide recommended sizes based on CFM and velocity: