A new turbulence model to calculate complex turbulent heat transfer in separating and reattaching flows is proposed. This new model is a modified version of the latest low-Reynolds-number two-equation heat-transfer model, in which the main improvement is achieved by introducing the Kolmogorov velocity scale, u(epsilon) equivalent to (V epsilon)(1/4), instead of the friction velocity u(tau), to account for the near-wall and low-Reynolds-number effects in both attached and detached hows. After investigating the characteristics of various time scales for the heat-transfer model, we adopted a composite time scale which gives weight to a shorter scale among the velocity- and temperature-field time scales. It is validated that the present model predicts quite accurately the turbulent heat transfer in separating and reattaching flows downstream of a backward-facing step, which involve most of the essential physics of complex turbulent heat transfer, under various conditions of flow Reynolds number and upstream boundary-layer thickness. In addition, the computational results have revealed several new mechanistic features of the turbulent heat transfer in separating and reattaching flows.