1. What is the Air Duct Sizing Equation?

The air duct sizing equation calculates the required cross-sectional area of a duct based on the 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 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 appropriate duct size.

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 while oversized ducts reduce air velocity.

4. Using the Calculator

Tips: Enter the required airflow in CFM and desired air velocity in feet per minute. Typical air velocities are 600-900 fpm for supply ducts and 500-700 fpm for return ducts.

5. Frequently Asked Questions (FAQ)

Q1: What is the recommended air velocity for residential systems?
A: For residential systems, 600-900 fpm is typical for supply ducts and 500-700 fpm for return ducts.

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

Q3: What happens if ducts are undersized?
A: Undersized ducts increase air velocity, causing noise, reduced airflow, and higher energy consumption due to increased static pressure.

Q4: Should I consider friction loss in duct sizing?
A: Yes, for complete system design, friction loss calculations should be performed using the Darcy-Weisbach equation or equivalent.

Q5: How does altitude affect duct sizing?
A: At higher altitudes, air is less dense, so CFM requirements may need adjustment. The basic equation remains valid but system design should account for altitude effects.