Air Duct Sizing Calculator

Duct Diameter Formula:

\[ \text{Diameter (in)} = \sqrt{\frac{\text{CFM} \times 1.1}{\text{Velocity}}} \]

Diameter (in):

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1. What is Air Duct Sizing?

Air duct sizing is the process of determining the appropriate diameter for HVAC ductwork based on airflow requirements (CFM) and desired air velocity. Proper duct sizing ensures efficient airflow, reduces energy consumption, and maintains optimal system performance.

2. How Does the Calculator Work?

The calculator uses the duct diameter formula:

\[ \text{Diameter (in)} = \sqrt{\frac{\text{CFM} \times 1.1}{\text{Velocity}}} \]

Where:

\( CFM \) — Cubic Feet per Minute (airflow volume)
\( Velocity \) — Feet per Minute (air speed)
\( 1.1 \) — Conversion factor for circular ducts

Explanation: The formula calculates the minimum duct diameter needed to handle the specified airflow at the desired velocity while maintaining efficient operation.

3. Importance of Proper Duct Sizing

Details: Correct duct sizing is crucial for HVAC system efficiency. Undersized ducts cause high static pressure, reduced airflow, and increased energy consumption. Oversized ducts lead to poor air distribution and noise issues.

4. Using the Calculator

Tips: Enter CFM (airflow volume in 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 does duct diameter affect system performance?
A: Smaller diameters increase velocity and static pressure, potentially reducing airflow. Larger diameters decrease velocity but improve airflow efficiency.

Q3: What factors influence duct sizing besides CFM and velocity?
A: Duct material, length, number of bends, static pressure, and system type all affect optimal duct sizing decisions.

Q4: Can this calculator be used for rectangular ducts?
A: This formula calculates circular duct diameter. For rectangular ducts, equivalent diameter calculations are needed based on aspect ratio.

Q5: What are common CFM requirements for different room sizes?
A: Bedrooms: 50-100 CFM, living rooms: 100-150 CFM, kitchens: 100-150 CFM, bathrooms: 50-100 CFM depending on size and usage.