1. What is the Duct Size Formula?

The duct size formula calculates the required cross-sectional area of a duct based on the required airflow (CFM) and desired air velocity. Proper duct sizing is essential for efficient HVAC system operation.

2. How Does the Calculator Work?

The calculator uses the duct size equation:

Where:

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

Explanation: The equation balances airflow requirements with velocity constraints to determine the minimum duct size needed.

3. Importance of Proper Duct Sizing

Details: Correct duct sizing ensures adequate airflow, minimizes pressure drop, reduces noise, and improves energy efficiency in HVAC systems.

4. Using the Calculator

Tips: Enter CFM based on room requirements and select appropriate velocity (typically 600-900 fpm for supply ducts, 500-700 fpm for return ducts).

5. Frequently Asked Questions (FAQ)

Q1: What is a typical air velocity for residential ducts?
A: For residential systems, 600-900 fpm is common for supply ducts, while 500-700 fpm is typical for return ducts.

Q2: How does duct shape affect sizing?
A: Round ducts are more efficient than rectangular ones. When using rectangular ducts, maintain similar cross-sectional area while considering space constraints.

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

Q4: Should I account for duct length?
A: This calculator provides the minimum area. For long duct runs, you may need to increase size to compensate for friction losses.

Q5: What about duct insulation?
A: Insulation thickness should be considered when determining final duct dimensions, especially in tight spaces.