Steady-state estimate of heat transfer through duct and insulation using Q = U · A · ΔT.
Includes inside & outside film resistances and insulation conduction.
About this duct heat gain / loss calculator
This free duct heat gain / loss calculator gives a steady-state estimate of the heat transferred through a run of ductwork and its insulation, using Q = U · A · ΔT. Enter the duct air temperature, the surrounding ambient temperature (ceiling void, plantroom or outdoors), the duct geometry and the insulation, and it returns the total heat transfer in watts, the loss per metre, the effective U-value and the insulated external surface area. It is aimed at UK building-services engineers and HVAC contractors sizing or checking supply and return ducts, and at anyone weighing up the benefit of adding insulation to an existing run.
How it works
The tool builds a series of thermal resistances from the duct air to the surroundings, the inside air film Rsi = 1/(hi·Aint), the insulation conduction Rcond, and the outside air film Rso = 1/(ho·Aext): then computes Q = ΔT / Rtotal, following the ASHRAE Handbook — Fundamentals approach to ducts and insulation. For circular ducts the conduction term uses the cylindrical form Rcond = ln(r₂/r₁)/(2π·k·L); for rectangular ducts it uses a planar average-area approximation, and flags a warning when the insulation thickness exceeds half the smallest duct side. Insulation presets cover mineral wool, phenolic foam, PIR/PUR and elastomeric (or a custom k value), and film coefficients are selectable for the inside and outside surfaces. An optional annual energy estimate applies a utilisation factor and operating hours, with an electrical figure derived from the system COP.
Frequently asked questions
Does it calculate heat gain or heat loss?
Both. By default the tool reads the direction from the sign of ΔT (ambient minus duct air), a warmer surrounding gives heat gain, a cooler one gives heat loss. The magnitude of Q always uses the absolute ΔT, so forcing the gain or loss label changes only how the result is described, never the calculated value.
Which standard does it follow?
The steady-state method: Q = ΔT / ΣR through the inside film, insulation conduction and outside film. Follows the ASHRAE Handbook — Fundamentals treatment of ducts and insulation. It is intended for design guidance only; always verify against your project specification with a qualified engineer.
How accurate is the rectangular duct result?
Rectangular ducts use a planar average-area approximation for the insulation, which is reliable for typical thicknesses but can under-predict heat flow once the insulation gets thick relative to the smaller duct side. When the thickness exceeds half the smallest side the calculator shows a warning and the conduction resistance should be treated as an upper bound.