Experiments were conducted with hot water in a horizontal 12.09-mm nominal diameter, 1.83-m long, double-pipe microfin tube heat exchanger, at effective Reynolds numbers in the range of 3573−12,621, and an average Prandtl number of 4.5. Smooth tube heat transfer and pressure drop measurements for similar flow conditions agreed satisfactorily with widely reported smooth tube correlations. Heat transfer measurements conducted at a linearly averaged, constant microfin tube wall temperature condition, and the corresponding isothermal pressure drop measurements were reduced to Nusselt numbers and friction factors based on the microfin tube effective diameter. A maximum heat transfer enhancement ratio (h_deff)_microfin/h_smooth of 2.07 was obtained, while the corresponding efficiency index (h_deff/f_deff)_microfin/(h/f)_smooth was 1.77, indicating that this particular microfin tube performed excellently in the developing turbulent flow regime. The maximum friction enhancement (f_deff)_microfin/f_smooth was 1.2. The key element in this work is the development of correlations (based on the effective diameter Reynolds number) corrected for entrance length effects for both heat transfer and friction pressure drop, and generation of experimental data, for transition flows in a microfin tube heat exchanger. Further, the thermal performance of such heat exchangers is quantified, based on the global measurements in the operating range of conventional HVAC equipment.