[en] Heat transfer results of an insulated oval duct generated by the conventional one-dimensional method that incorporates an approximate oval perimeter equations are not accurate. The greater the long to short axes ratio is, the larger the error becomes. More reliable results, judged by comparing with accurate numerical simulations, can be obtained based on accurate oval perimeters, calculated by an integral method combined with a one-dimensional PWTR model or 91-CPWTR model. In practical situations where an oval's long to short axes ratio a/b is less than or equal to 3/1, accurate and reliable solutions (errors within ±2% while t/R₂ ≤ 2.0) can be obtained by the one-dimensional PWTR model. However, when the long to short axes ratio is greater than 3/1, better solutions can be achieved by the one-dimensional 91-CPWTR model. For most practical insulated thickness situations (t/R₂ < 0.5), applying the one-dimensional PWTR model to insulated oval ducts with the long to short axes ratio a/b less than or equal to 5/1 results in errors within ±1%