The heat-transfer characteristics of an insulated and non-insulated long rectangular duct considering heat radiation are analysed by using the one-dimensional Plane Wedge Thermal Resistance (PWTR) model and Plate Thermal Resistance (PTR) model in this study. It is found that the errors generated by the PWTR model are all positive and the errors generated by the PTR model are all negative. Thus, the Combined Plate Wedge Thermal Resistance (CPWTR) model generated by paralleling PWTR and PTR models with the proportion factors of 0.6 vs. 0.4 (64-CPWTR model) or 0.7 vs. 0.3 (73-CPWTR model) can neutralize the positive and negative errors and obtain very accurate results in comparison with the two-dimensional numerical solutions analysed by the CFD software. Most errors generated by the one-dimensional 64-CPWTR model are most within 1.5% for practical sizes and practical insulated thickness (t/R2≦1); most errors
generated by the one-dimensional 73-CPWTR model are most within 2.2% for practical sizes and smaller errors for thicker insulated thickness (1<t/R2≦2). For the results obtained by the same method as the present study except neglecting heat radiation, it is found that neglecting the heat radiation effect is likely to produce very big errors of non-insulated and thin-insulated ducts in situations of ambient air with low external convection heat coefficients and larger surface emissivity.