1. What is the Duct Size Formula?

The duct size formula calculates the required cross-sectional area of an air duct based on the 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 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 formula 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 are wasteful and may cause velocity problems.

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 HVAC system requirements
• Select velocity appropriate for your application

5. Frequently Asked Questions (FAQ)

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

Q2: How do I convert square feet to round duct diameter?
A: Diameter = 2 × √(Area/π). Our calculator provides this conversion automatically.

Q3: What happens if ducts are too small?
A: Undersized ducts cause increased static pressure, reduced airflow, higher energy consumption, and potential noise issues.

Q4: Are rectangular ducts calculated differently?
A: The area calculation is the same, but aspect ratio (width:height) affects friction loss. Our calculator gives the area which can be used for any shape.

Q5: What's the maximum recommended velocity?
A: Generally 1500 fpm for main ducts and 900 fpm for branch ducts in residential systems, but varies by application and noise requirements.