An experimental determination of the steady-state forced convection from the internal surfaces of a horizontal, uniformly heated electrically, equilateral-triangle cross-sectioned duct with sharp corners has been undertaken. The average Nusselt number <(Nu)over bar (D)>, using the hydraulic diameter as the characteristic physical dimension, is a constant if the flow in the duct is laminar, bur a function of the air’s Reynolds number, when the flow is turbulent. Non-dimensional correlations, which can be used for predicting the value of the steady-state rate of convective hear transfer from such a triangular duct into the airflow, have been deduced, viz. <(Nu)over bar (D)> = 3.25 for laminar flows, i.e. Re-D < 1450; and <(Nu)over bar (D)> = 0.012 Re-D(0.83) for turbulent flows, i.e. Re-D greater than or equal to 1450.