Rectangular Duct Size Calculator

Duct Size Equations:

\[ A = \frac{CFM}{V} \] \[ \text{Width} \times \text{Height} = A \]

Airflow (CFM):

cubic feet per minute

Air Velocity:

feet per minute

Aspect Ratio (Width:Height):

Cross-sectional Area:

Duct Width:

Duct Height:

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1. What is the Rectangular Duct Size Calculator?

The Rectangular Duct Size Calculator determines the appropriate dimensions for HVAC ductwork based on airflow requirements and desired air velocity. It helps ensure efficient air distribution while minimizing pressure drops and noise.

2. How Does the Calculator Work?

The calculator uses these fundamental equations:

\[ A = \frac{CFM}{V} \] \[ \text{Width} \times \text{Height} = A \]

Where:

\( A \) — Cross-sectional area (square feet)
\( CFM \) — Airflow in cubic feet per minute
\( V \) — Air velocity in feet per minute
Width — Duct width (inches)
Height — Duct height (inches)

Explanation: The calculator first determines the required cross-sectional area based on airflow and velocity, then calculates dimensions according to your selected aspect ratio.

3. Importance of Proper Duct Sizing

Details: Correct duct sizing is critical for efficient HVAC system operation. Undersized ducts increase velocity, causing noise and pressure drops. Oversized ducts reduce velocity, potentially leading to poor air distribution and increased installation costs.

4. Using the Calculator

Tips:

Typical air velocities: 600-900 fpm for supply, 500-700 fpm for return
Common aspect ratios range from 1:1 (square) to 4:1 (flat rectangular)
Higher aspect ratios save vertical space but may require more material

5. Frequently Asked Questions (FAQ)

Q1: What's the recommended air velocity for residential systems?
A: Typically 600-900 fpm for supply ducts and 500-700 fpm for return ducts in residential applications.

Q2: How does aspect ratio affect duct performance?
A: Higher aspect ratios (more rectangular ducts) can save space but may require more material and have slightly higher friction losses than square ducts of equivalent area.

Q3: Should I round up or down the calculated dimensions?
A: Round up to the nearest standard duct size (typically in 1-inch increments) to ensure adequate airflow capacity.

Q4: What about duct insulation?
A: Remember to account for insulation thickness when determining final installed dimensions, especially in tight spaces.

Q5: When should I consider using round ducts instead?
A: Round ducts are more efficient (less surface area for same airflow) and often preferred when space allows, though rectangular ducts are common where space is constrained.