1. What is the Ductwork Sizing Equation?

The duct sizing equation calculates the required cross-sectional area of heating ducts based on 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 sizing 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 appropriate duct size.

3. Importance of Proper Duct Sizing

Details: Correct duct sizing ensures proper airflow, minimizes energy loss, reduces noise, and maintains system efficiency. Undersized ducts increase static pressure while oversized ducts reduce air velocity.

4. Using the Calculator

Tips: Enter CFM (based on heating load calculations) and desired air velocity (typically 600-900 fpm for supply ducts). Both values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: What is a typical air velocity for residential heating ducts?
A: Supply ducts typically use 600-900 fpm, while return ducts use 500-700 fpm for quiet operation.

Q2: How does duct shape affect sizing?
A: Round ducts are more efficient than rectangular. When using rectangular ducts, maintain aspect ratio below 4:1.

Q3: What CFM is needed per square foot?
A: This varies by climate and insulation, but a rough estimate is 1 CFM per square foot of floor area.

Q4: Should I account for duct losses?
A: Yes, duct losses (typically 10-30%) should be added to the calculated CFM requirements.

Q5: What about duct material?
A: Different materials have different friction rates. Flexible ducts require larger sizes than smooth metal ducts for equivalent airflow.