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HVAC Duct Sizing Formula:
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The HVAC duct sizing formula A = CFM/V calculates the cross-sectional area (A) of a duct needed to handle a specific airflow (CFM) at a given velocity (V). This is fundamental in HVAC system design to ensure proper airflow distribution.
The calculator uses the duct sizing equation:
Where:
Explanation: This formula ensures that ducts are properly sized to maintain appropriate air velocity, balancing efficiency and noise considerations.
Details: Correct duct sizing is crucial for efficient HVAC system operation. Undersized ducts increase static pressure and reduce airflow, while oversized ducts can lead to air distribution problems and increased installation costs.
Tips: Enter CFM (cubic feet per minute) and velocity (feet per minute). Both values must be positive numbers. Typical residential duct velocities range from 600-900 ft/min for supply and 500-700 ft/min for return air.
Q1: What is a typical CFM requirement for residential HVAC?
A: Residential systems typically require 400-600 CFM per ton of cooling capacity, but this varies based on climate and building characteristics.
Q2: What are recommended air velocities for different applications?
A: Supply ducts: 600-900 ft/min, Return ducts: 500-700 ft/min, Low-velocity systems: 400-600 ft/min, High-velocity systems: 1000-2000 ft/min.
Q3: How do I convert round duct area to diameter?
A: For round ducts: Diameter = 2 × √(Area/π). The calculator provides area; you may need to convert to equivalent round diameter for practical application.
Q4: Does this formula work for rectangular ducts?
A: Yes, the formula calculates the required cross-sectional area regardless of duct shape. For rectangular ducts, you would determine appropriate width and height dimensions that provide the calculated area.
Q5: What factors affect proper duct sizing besides CFM and velocity?
A: Additional considerations include duct material roughness, system static pressure, duct length, number of bends, and temperature differences.