The Duct Heat Loss Calculator measures how much heat escapes from a duct. It looks at the duct’s size, the temperature difference, and how well heat moves through the duct walls. You get a number in watts or BTUs per hour, showing the heat loss. This is great for real-life decisions, like choosing insulation, sizing heaters, or saving energy.
This calculator ensures your system works well, keeping air at the right temperature when it reaches its destination. It’s reliable for important tasks, like lowering heating bills or improving comfort. Ready to see how it’s done? Let’s check out the formula next.
Formula for Duct Heat Loss
The formula for calculating heat loss from a duct is simple:
Q = U × A × (Ti - Ta)
Where:
- Q = Heat loss (Watts or BTU/hr)
- U = Overall heat transfer coefficient (W/m²·K or BTU/hr·ft²·°F)
- A = Surface area of the duct (m² or ft²)
- Ti = Temperature inside the duct (°C or °F)
- Ta = Ambient temperature outside the duct (°C or °F)
Surface Area for Rectangular Ducts
For rectangular ducts, calculate the surface area like this:
A = 2(W + H) × L
Where:
- W = Width of the duct (m or ft)
- H = Height of the duct (m or ft)
- L = Length of the duct (m or ft)
These formulas come from heat transfer science. The U value depends on duct material and insulation—common values are 1-5 W/m²·K or 0.2-1 BTU/hr·ft²·°F. Now, let’s make it easier with a table.
Quick Reference Table for Duct Heat Loss
Why calculate every time? This table shows heat loss for common duct setups with a U value of 2 BTU/hr·ft²·°F. It’s a fast way to check without doing math.
| Duct Size (W×H×L, ft) | Ti (°F) | Ta (°F) | Surface Area (ft²) | Heat Loss (BTU/hr) |
|---|---|---|---|---|
| 1×0.5×10 | 80 | 50 | 30 | 1,800 |
| 2×1×15 | 75 | 40 | 90 | 6,300 |
| 1.5×1×20 | 70 | 45 | 100 | 5,000 |
How to Use the Table
- Find your duct size and temperatures.
- Check the heat loss value.
- Adjust for different U values or units if needed.
This table helps with searches like “heat loss for 2x1 duct.” For exact results, use the formula. Next, let’s try an example.
Example of Duct Heat Loss Calculator
Suppose you have a rectangular duct that’s 1 foot wide, 0.5 feet high, and 12 feet long. The air inside is 75°F, the outside air is 40°F, and the U value is 2 BTU/hr·ft²·°F. You want to know the heat loss. Here’s how to do it:
- Calculate the surface area:
A = 2(W + H) × L
A = 2(1 + 0.5) × 12 = 2 × 1.5 × 12 = 36 ft² - Find the temperature difference:
Ti - Ta = 75 - 40 = 35°F - Plug into the formula:
Q = U × A × (Ti - Ta)
Q = 2 × 36 × 35 = 2,520 BTU/hr
So, the heat loss is 2,520 BTU per hour. This matches engineering standards and helps you plan insulation or heating.
Most Common FAQs
Heat loss affects how much energy you need to keep air warm or cool—less loss means lower costs.
A lower U value (like 1-2) means better insulation and less heat loss—check your duct material.
Yes, just use the right surface area formula for round ducts (A = π × D × L) with the same Q formula.