1. What is the Duct Size Formula?

The duct size formula (A = CFM/V) calculates the cross-sectional area needed for heating and cooling ducts based on airflow rate 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 size 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 optimal duct size.

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 while oversized ducts waste energy.

4. Using the Calculator

Tips: Enter CFM based on your HVAC system requirements and select appropriate velocity (typically 600-900 fpm for supply, 500-700 fpm for return). Both values must be positive.

5. Frequently Asked Questions (FAQ)

Q1: What are typical air velocities for ducts?
A: Supply ducts: 600-900 fpm, Return ducts: 500-700 fpm, Low-velocity systems: 300-500 fpm.

Q2: How does duct shape affect sizing?
A: Round ducts are more efficient than rectangular. Equivalent round diameter can be calculated for rectangular ducts.

Q3: What factors affect CFM requirements?
A: Room size, insulation, window area, climate, and heat load calculations determine required CFM.

Q4: Should I account for duct friction loss?
A: Yes, professional designs include friction rate (typically 0.1 in. w.c. per 100 ft) which affects sizing.

Q5: What about duct material?
A: Material affects friction but not basic sizing. Flexible ducts require higher CFM due to greater resistance.