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Calculation Machine

Duct Heat Gain Calculator

Cooling season heat gain into ducts.

Reviewed by the Calculation Machine team · How we check our maths

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.

The formula

Q (BTU/hr) = surface area x ΔT / R-value

Worked example

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

Result

  • Duct surface area83.8 sq ft
  • Heat gain into duct1,047 BTU/hr
  • Equivalent cooling capacity lost0.09 tons

How to use the Duct Heat Gain Calculator

  1. 1Enter duct size, length and insulation R-value.
  2. 2Enter the hot attic temperature and the cold supply air temperature.
  3. 3The result shows how much of your AC capacity is wasted reheating the duct's contents.

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