1. What is the Duct Size Formula?

The duct size formula calculates the required cross-sectional area of an HVAC 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 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 ensures adequate air delivery while maintaining proper velocity to minimize noise and pressure drop.

3. Importance of Proper Duct Sizing

Details: Correct duct sizing prevents airflow problems, reduces energy consumption, minimizes noise, and ensures proper system balancing and performance.

4. Using the Calculator

Tips:

• Typical residential air velocity: 600-900 ft/min
• Commercial systems: 1000-2000 ft/min
• Low-velocity systems: 400-600 ft/min
• High-velocity systems: 2000-3500 ft/min

5. Frequently Asked Questions (FAQ)

Q1: What's the recommended velocity for residential systems?
A: 600-900 ft/min for supply ducts, 500-700 ft/min for return ducts.

Q2: How do I convert square feet to round duct diameter?
A: Diameter = 2 × √(Area/π). For rectangular ducts, maintain aspect ratio below 4:1.

Q3: What happens if ducts are oversized?
A: Reduced air velocity can lead to poor air distribution, stratification, and increased installation costs.

Q4: What about undersized ducts?
A: Excessive velocity causes noise, increased pressure drop, and reduced system efficiency.

Q5: Should I account for duct friction losses?
A: Yes, for precise design use the equivalent length method or ductulator tools that consider friction.