1. What is the HVAC Duct Size Formula?

The basic duct sizing formula calculates the required cross-sectional area of a 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 duct area formula:

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 determines the minimum duct area needed to move a given airflow at a specified velocity without excessive pressure drop or noise.

3. Importance of Proper Duct Sizing

Details: Correct duct sizing ensures proper airflow distribution, maintains system efficiency, reduces noise, and prevents pressure imbalances in the HVAC system.

4. Using the Calculator

Tips: Enter CFM (typically 400 CFM per ton of cooling) and desired velocity (typically 600-900 FPM for supply ducts). All values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: What are typical air velocities for different duct types?
A: Supply ducts: 600-900 FPM, Return ducts: 500-700 FPM, Branch ducts: 500-600 FPM.

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

Q3: What about static pressure?
A: This simple formula doesn't account for pressure drop. Detailed duct design considers friction rate (typically 0.1" w.g. per 100 ft).

Q4: How to convert area to round duct diameter?
A: Diameter = 2 × √(Area/π). For rectangular ducts, maintain aspect ratio below 4:1.

Q5: When should I use higher velocities?
A: Higher velocities (up to 1500 FPM) may be used in main ducts where space is limited, but noise increases.