How To Calculate Air Duct Size

Duct Area Formula:

\[ \text{Duct Area (ft²)} = \frac{\text{CFM (ft³/min)}}{\text{Velocity (fpm)}} \]

Duct Area:

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1. What is Duct Sizing?

Duct sizing is the process of determining the appropriate cross-sectional area of air ducts in HVAC systems based on airflow requirements and velocity constraints. Proper duct sizing ensures efficient air distribution and system performance.

2. How Does the Calculator Work?

The calculator uses the fundamental duct sizing formula:

\[ \text{Duct Area (ft²)} = \frac{\text{CFM (ft³/min)}}{\text{Velocity (fpm)}} \]

Where:

\( CFM \) — Cubic Feet per Minute (airflow rate)
\( Velocity \) — Feet per Minute (air velocity through duct)
\( Duct Area \) — Cross-sectional area in square feet

Explanation: This formula calculates the required duct area to handle a specific airflow at a given velocity, ensuring proper system performance and efficiency.

3. Importance of Proper Duct Sizing

Details: Correct duct sizing is crucial for maintaining proper airflow, minimizing pressure drops, reducing noise, optimizing energy efficiency, and ensuring even temperature distribution throughout the building.

4. Using the Calculator

Tips: Enter CFM (cubic feet per minute) and desired air velocity in feet per minute. Typical residential velocities range from 600-900 fpm for supply ducts and 500-700 fpm for return ducts.

5. Frequently Asked Questions (FAQ)

Q1: What is the recommended velocity for residential ducts?
A: For supply ducts: 600-900 fpm; for return ducts: 500-700 fpm. Lower velocities reduce noise but require larger ducts.

Q2: How do I convert duct area to duct dimensions?
A: For rectangular ducts: Area = Width × Height. For round ducts: Area = π × (Diameter/2)². Common aspect ratios are 1:1 to 4:1 for rectangular ducts.

Q3: What happens if ducts are undersized?
A: Undersized ducts cause increased air velocity, higher pressure drops, reduced airflow, increased noise, and higher energy consumption.

Q4: Are there industry standards for duct sizing?
A: Yes, standards include ACCA Manual D, ASHRAE guidelines, and SMACNA standards which consider friction loss and static pressure.

Q5: How does duct material affect sizing?
A: Different materials have different friction factors. Flexible ducts typically require larger sizes than sheet metal due to higher friction losses.