1. What is the AC Duct Size Formula?

The duct size formula calculates the required cross-sectional area of an air conditioning 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 basic 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 the minimum duct size needed.

3. Importance of Proper Duct Sizing

Details: Correct duct sizing ensures proper airflow, minimizes energy loss, reduces noise, and maintains system efficiency. Undersized ducts increase static pressure and reduce airflow, while oversized ducts can lead to poor air distribution.

4. Using the Calculator

Tips:

• Typical air velocities: 600-900 fpm for main ducts, 500-700 fpm for branch ducts
• Residential systems typically use 0.1-0.15 inches of water column static pressure
• Commercial systems may use higher velocities (up to 2000 fpm) and pressures

5. Frequently Asked Questions (FAQ)

Q1: What's the recommended velocity for residential AC ducts?
A: For quiet operation, 600-900 fpm for main ducts and 500-700 fpm for branch ducts is recommended.

Q2: How does duct shape affect sizing?
A: Round ducts are more efficient than rectangular ones. When using rectangular ducts, aim for an aspect ratio no greater than 4:1.

Q3: What about duct friction loss?
A: This calculator provides basic sizing. For complete design, friction loss charts or ductulators should be used to account for duct length and material.

Q4: How does altitude affect duct sizing?
A: At higher altitudes, air is less dense, so CFM values should be increased by about 3% per 1000 feet above sea level.

Q5: What's the relationship between duct size and static pressure?
A: Smaller ducts increase static pressure and reduce airflow. The system fan must be able to overcome the total static pressure of the duct system.