Cfm Duct Calculator

CFM Duct Calculator

Rectangular Round

function toggleDuctInputs() { var ductShape = document.getElementById("ductShape").value; var rectangularDiv = document.getElementById("rectangularInputs"); var roundDiv = document.getElementById("roundInputs"); if (ductShape === "rectangular") { rectangularDiv.style.display = "block"; roundDiv.style.display = "none"; } else { rectangularDiv.style.display = "none"; roundDiv.style.display = "block"; } } function calculateCFM() { var ductShape = document.getElementById("ductShape").value; var airVelocity = parseFloat(document.getElementById("airVelocity").value); var ductAreaSqFt; var resultDiv = document.getElementById("result"); if (isNaN(airVelocity) || airVelocity <= 0) { resultDiv.innerHTML = "Please enter a valid positive Air Velocity."; resultDiv.style.color = "#dc3545"; resultDiv.style.backgroundColor = "#f8d7da"; return; } if (ductShape === "rectangular") { var ductWidth = parseFloat(document.getElementById("ductWidth").value); var ductHeight = parseFloat(document.getElementById("ductHeight").value); if (isNaN(ductWidth) || ductWidth <= 0 || isNaN(ductHeight) || ductHeight <= 0) { resultDiv.innerHTML = "Please enter valid positive Duct Width and Height."; resultDiv.style.color = "#dc3545"; resultDiv.style.backgroundColor = "#f8d7da"; return; } ductAreaSqFt = (ductWidth * ductHeight) / 144; // Convert square inches to square feet } else { // round var ductDiameter = parseFloat(document.getElementById("ductDiameter").value); if (isNaN(ductDiameter) || ductDiameter <= 0) { resultDiv.innerHTML = "Please enter a valid positive Duct Diameter."; resultDiv.style.color = "#dc3545"; resultDiv.style.backgroundColor = "#f8d7da"; return; } var radiusInches = ductDiameter / 2; ductAreaSqFt = (Math.PI * radiusInches * radiusInches) / 144; // Convert square inches to square feet } var cfm = ductAreaSqFt * airVelocity; resultDiv.innerHTML = "Calculated CFM: " + cfm.toFixed(2) + " CFM"; resultDiv.style.color = "#28a745"; resultDiv.style.backgroundColor = "#e9f7ef"; } // Initialize the display based on default selection window.onload = toggleDuctInputs;

Understanding CFM and Duct Sizing

CFM stands for Cubic Feet per Minute, and it's a critical measurement in HVAC (Heating, Ventilation, and Air Conditioning) systems. It quantifies the volume of air moving through a space or ductwork in one minute. Proper CFM is essential for maintaining indoor air quality, comfort, and the efficient operation of heating and cooling equipment.

Why is CFM Important?

• Comfort: Too little CFM can lead to hot or cold spots, while too much can create uncomfortable drafts.
• Efficiency: Correct airflow ensures your HVAC system isn't working harder than it needs to, saving energy and extending equipment life.
• Air Quality: Adequate CFM helps in removing pollutants, allergens, and stale air, bringing in fresh, conditioned air.
• Equipment Performance: HVAC units are designed to operate within specific CFM ranges. Incorrect airflow can lead to overheating, freezing coils, and premature failure.

The Relationship Between CFM, Duct Size, and Air Velocity

The fundamental relationship is straightforward: CFM = Duct Area (in square feet) × Air Velocity (in feet per minute).

• Duct Area: This is the cross-sectional area of your ductwork. Larger ducts can handle more air at lower velocities.
• Air Velocity: This is how fast the air is moving through the duct. Higher velocities can move more air through smaller ducts, but can also lead to increased noise and static pressure.

This calculator helps you determine the CFM given your duct's dimensions and the air's velocity. This is particularly useful for:

• System Design: Sizing new ductwork for a new HVAC installation.
• Troubleshooting: Diagnosing airflow issues in existing systems.
• Performance Verification: Checking if an existing system is delivering the expected airflow.

Typical Air Velocities

The ideal air velocity depends on the application and location within the duct system. Here are some general guidelines:

• Residential Supply Ducts: 600-900 FPM (Feet per Minute)
• Residential Return Ducts: 400-700 FPM
• Commercial Supply Ducts (Main): 1000-2200 FPM
• Commercial Return Ducts (Main): 800-1800 FPM
• Branch Ducts: Generally lower than main ducts to reduce noise.

Exceeding recommended velocities can lead to excessive noise, increased static pressure (making your fan work harder), and higher energy consumption. Conversely, velocities that are too low can result in insufficient airflow and poor temperature distribution.

How to Use This Calculator

1. Select Duct Shape: Choose whether your duct is rectangular or round.
2. Enter Dimensions:
   • For Rectangular: Input the Width and Height in inches.
   • For Round: Input the Diameter in inches.
3. Enter Air Velocity: Input the air speed in Feet per Minute (FPM).
4. Click "Calculate CFM": The calculator will instantly display the total Cubic Feet per Minute.

Examples of CFM Calculation

Let's look at a couple of practical examples:

Example 1: Rectangular Duct

• Duct Shape: Rectangular
• Duct Width: 14 inches
• Duct Height: 10 inches
• Air Velocity: 800 FPM
• Calculation:
  • Duct Area (sq inches) = 14 in * 10 in = 140 sq in
  • Duct Area (sq feet) = 140 sq in / 144 = 0.9722 sq ft
  • CFM = 0.9722 sq ft * 800 FPM = 777.78 CFM

Example 2: Round Duct

• Duct Shape: Round
• Duct Diameter: 12 inches
• Air Velocity: 750 FPM
• Calculation:
  • Radius (inches) = 12 in / 2 = 6 in
  • Duct Area (sq inches) = π * (6 in)^2 = 3.14159 * 36 = 113.097 sq in
  • Duct Area (sq feet) = 113.097 sq in / 144 = 0.7854 sq ft
  • CFM = 0.7854 sq ft * 750 FPM = 589.05 CFM

Using this calculator, you can quickly determine the airflow capacity of your ductwork, aiding in efficient HVAC system design and maintenance.