1. What is the Duct Size Formula?

The duct size formula calculates the required cross-sectional area of an air conditioning duct based on the airflow rate (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 rate (cubic feet per minute)
• \( V \) — Air velocity (feet per minute)

Explanation: The equation balances airflow requirements with velocity to determine the appropriate duct size for efficient air distribution.

3. Importance of Proper Duct Sizing

Details: Correct duct sizing ensures proper airflow, reduces energy consumption, minimizes noise, and maintains comfort levels throughout the building.

4. Using the Calculator

Tips: Enter CFM (typically 400 CFM per ton of cooling for residential systems) and desired velocity (typically 600-900 FPM for supply ducts). All values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: What is the typical air velocity for ducts?
A: Supply ducts typically use 600-900 FPM, return ducts 500-700 FPM, and main ducts up to 1200 FPM for commercial systems.

Q2: How does duct shape affect sizing?
A: Round ducts are more efficient than rectangular ones. When using rectangular ducts, maintain an aspect ratio no greater than 4:1.

Q3: What happens if ducts are undersized?
A: Undersized ducts cause reduced airflow, increased noise, higher energy consumption, and potential equipment damage.

Q4: Should I account for duct losses?
A: Yes, this calculator provides theoretical size. Actual design should account for friction losses, fittings, and duct length.

Q5: What about duct insulation?
A: Insulation affects thermal performance but not the basic sizing calculation. Always insulate ducts in unconditioned spaces.