Duct Size Equation:
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The duct size equation calculates the required cross-sectional area of a duct based on airflow (CFM) and air velocity. Proper duct sizing is essential for efficient HVAC system operation and balanced airflow throughout the home.
The calculator uses the duct size equation:
Where:
Explanation: The equation balances airflow requirements with velocity constraints to determine the appropriate duct size for residential HVAC systems.
Details: Correct duct sizing ensures proper airflow, system efficiency, comfort, and noise control. Undersized ducts increase static pressure and reduce airflow, while oversized ducts can lead to poor air distribution and increased installation costs.
Tips: Enter CFM based on your HVAC system capacity and room requirements. Typical velocity ranges are 600-900 fpm for supply ducts and 500-700 fpm for return ducts in residential systems.
Q1: What are typical CFM requirements for rooms?
A: Bedrooms typically need 50-100 CFM, living rooms 100-150 CFM, and kitchens 75-100 CFM per 100 square feet.
Q2: What velocity should I use for calculations?
A: For supply ducts, 600-900 fpm is common. For return ducts, 500-700 fpm is typical. Lower velocities reduce noise but require larger ducts.
Q3: How do I convert square feet to round duct diameter?
A: Diameter = 2 × √(Area/π). A 0.2 sq ft area would be about a 6-inch round duct (2 × √(0.2/3.1416) ≈ 0.5 ft or 6 inches).
Q4: Does duct material affect sizing?
A: Material affects friction loss but not the basic size calculation. You may need to adjust for different materials' roughness factors.
Q5: Should I account for duct length?
A: This calculator provides the basic size. For long duct runs, you may need to increase size to compensate for friction losses.