1. What is the Duct Sizing Equation?

The duct sizing equation \( A = \frac{CFM}{V} \) calculates the required cross-sectional area of an HVAC duct based on airflow rate and desired air velocity. This fundamental equation helps ensure proper air distribution in residential HVAC systems.

2. How Does the Calculator Work?

The calculator uses the basic duct sizing equation:

Where:

• \( A \) — Duct cross-sectional area (square feet)
• \( CFM \) — Airflow rate (cubic feet per minute)
• \( V \) — Air velocity (feet per minute)

Explanation: The equation balances airflow requirements with velocity constraints to determine appropriate duct dimensions.

3. Importance of Proper Duct Sizing

Details: Correct duct sizing is essential for efficient HVAC operation, proper air distribution, noise control, and energy efficiency. Undersized ducts increase static pressure and reduce airflow, while oversized ducts can lead to poor air distribution and increased costs.

4. Using the Calculator

Tips:

• Typical residential air velocity: 600-900 fpm for supply, 500-700 fpm for return
• CFM requirements vary based on room size and HVAC capacity
• Always consult HVAC design manuals for complete system design

5. Frequently Asked Questions (FAQ)

Q1: What's the typical air velocity for residential ducts?
A: Supply ducts typically use 600-900 fpm, while return ducts use 500-700 fpm for optimal balance between noise and efficiency.

Q2: How do I determine required CFM for a room?
A: CFM depends on room size, heat load, and HVAC capacity. A common rule is 1 CFM per square foot of floor area, but professional calculations are recommended.

Q3: What about rectangular ducts?
A: For rectangular ducts, use the area to determine equivalent dimensions (e.g., 0.25 sq ft could be 6"x6" or 8"x4.5").

Q4: Does duct material affect sizing?
A: Material affects friction loss but not the basic area calculation. Different materials may require adjustments for equivalent friction rates.

Q5: Should I account for duct length?
A: This calculator provides the basic area. Long duct runs may require larger sizes to compensate for pressure drop, which requires more detailed calculations.