Duct Heat Gain Calculator
Cooling season heat gain into ducts.
Enter your values
ⓘ Live results
Results
- Duct surface area
- 83.8 sq ft
- Heat gain into duct
- 1,047 BTU/hr
- Equivalent cooling capacity lost
- 0.09 tons
Quick answer
Supply duct heat gain in a hot attic follows the same Q = A x ΔT / R relationship as heat loss, but adds unwanted heat to cooled air.
Q (BTU/hr) = surface area x ΔT / R-value
How to use the Duct Heat Gain Calculator
- 1Enter duct size, length and insulation R-value.
- 2Enter the hot attic temperature and the cold supply air temperature.
- 3The result shows how much of your AC capacity is wasted reheating the duct's contents.
Worked example
Using the values the calculator loads with:
Inputs
- Duct diameter8 in
- Duct length40 ft
- Insulation R-value6 R
- Attic temperature130 °F
- Supply air temperature55 °F
Results
- Duct surface area83.8 sq ft
- Heat gain into duct1,047 BTU/hr
- Equivalent cooling capacity lost0.09 tons
Frequently asked questions
Why does this matter more for AC than heating?
The temperature difference between a 130°F attic and 55°F supply air can be larger than typical winter conditions, driving high gain.
Best fix?
Move ducts into conditioned space where possible, or add insulation and seal leaks if they must stay in the attic.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.