1. What is the Duct Size Formula?

The duct size formula calculates the required cross-sectional area of HVAC ducts for residential systems based on airflow rate and air velocity. Proper duct sizing is essential for efficient HVAC system operation.

2. How Does the Calculator Work?

The calculator uses the 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 optimal duct size.

3. Importance of Proper Duct Sizing

Details: Correct duct sizing ensures proper air distribution, maintains system efficiency, reduces noise, and prevents pressure imbalances in the HVAC system.

4. Using the Calculator

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

5. Frequently Asked Questions (FAQ)

Q1: What are typical air velocities for residential ducts?
A: Supply ducts: 600-900 fpm, Return ducts: 500-700 fpm, Branch ducts: 400-600 fpm.

Q2: How do I convert duct area to dimensions?
A: For rectangular ducts, common aspect ratios are 1:1 (square) to 1:4 (flat). Round ducts use diameter = 2 × √(area/π).

Q3: What CFM values are typical for residential systems?
A: Generally 350-400 CFM per ton of cooling, with 1 ton ≈ 12,000 BTU/hr.

Q4: Should I include safety factors in my calculations?
A: Professional designs often include 10-20% safety margin for future expansion and system losses.

Q5: Are there printable duct sizing tables available?
A: Yes, many HVAC references provide printable duct sizing tables based on this formula for common scenarios.