Duct Heat Gain Calculation

Results

Calculator inputs

Formula used

The basic sensible heat-gain relation is Q = U × A × ΔT. Here, Q is heat gain, U is overall heat-transfer coefficient, A is external duct area, and ΔT is the ambient-to-air temperature difference.

Overall resistance is estimated using R = 1/h_i + t/k + 1/h_o, unless a direct R-value or U-value is supplied. Then U = 1/R.

Air temperature rise uses ΔT_air = Q/(ṁc_p), where ṁ = ρV̇. Optional solar heat is approximated as absorbed irradiance over exposed surface area.

How to use this calculator

1. Select SI or Imperial display units.
2. Choose the duct shape and enter dimensions.
3. Enter duct length, temperatures, and airflow.
4. Select insulation or enter custom conductivity.
5. Adjust film coefficients when better values are known.
6. Enable solar exposure for outdoor or roof ducts.
7. Select Calculate, then review results and steps.
8. Use CSV or PDF to save the calculation.

Example reference data

Duct heat gain explained

Duct heat gain occurs when air travels through a duct surrounded by warmer conditions. Heat moves through the duct wall and insulation toward the cooler supply air. The amount transferred depends on temperature difference, exposed surface area, insulation resistance, and surface heat-transfer conditions. Long ducts in hot attics often gain much more heat than short ducts inside conditioned rooms.

The overall U-value combines several thermal resistances into one practical coefficient. A lower U-value means better resistance to heat transfer. Insulation thickness usually has the strongest influence because its resistance equals thickness divided by thermal conductivity. Materials with lower conductivity provide more resistance for the same thickness. Film coefficients represent convection at the inner and outer surfaces. These values can vary with air speed, orientation, and surrounding airflow.

Airflow also matters because the same heat gain affects low airflow more strongly. The calculator converts volumetric airflow into mass flow using air density. It then estimates temperature rise from the added heat and air specific heat. This helps evaluate whether duct gains could raise supply temperature enough to reduce cooling performance.

Outdoor ducts may also receive solar radiation. The optional solar mode estimates absorbed solar energy using irradiance, surface absorptivity, exposure fraction, and duct area. This simplified treatment helps compare shaded and exposed installations. Detailed design may require weather data, radiation angle, duct surface temperature, and transient analysis.

Use calculated results as engineering estimates rather than certified design values. Real systems can include joints, leakage, thermal bridges, compression, moisture, and imperfect insulation. Measurements or detailed simulation are preferable for critical equipment sizing. Regularly comparing insulated and uninsulated cases can reveal where modest upgrades provide meaningful energy savings, improve delivered-air temperatures, and reduce cooling-system loads without changing the duct dimensions or airflow controls significantly. This tool supports practical preliminary duct heat-transfer comparisons effectively.

Frequently asked questions