To be used in the market, thermal and friction characteristics of newly launched heat exchangers must be acknowledged, usually by experimental methods. Steady-state Kays and London is considered the classical experimental technique. In this setup, one of the heat exchanger streams is usually vapor, to provide controlled known heat transfer and temperature conditions, while the other stream is subjected to different flow rates, for the determination of the equipment thermal and pressure drop behaviors. Large expensive industrial boilers are usually used, resulting in difficulties in stabilizing and controlling vapor temperatures. In the present work, a new experimental setup, based on the above-mentioned technique, is proposed for determining the heat transfer characteristics of compact heat exchangers. The boiler vapor flow is substituted by the working fluid (water in vapor state) of a two-phase thermosyphon. Being smaller and much more flexible, this technology allows for easy control of the vapor temperature, while providing uniform temperature distribution along one of the heat exchanger streams, which is difficult to obtain with the classical procedure. In the proposed apparatus, the “known side” of the heat exchanger takes the role of the condenser of the thermosyphon. Two well-known heat exchanger cores, composed by circular and square cross section channels, were used to validate the proposed arrangement, considering the ranges: 2200 < Re < 8000 and 850 < Re < 2800, respectively. The wall temperatures were tested in the range 120 °C to 220 °C with a precision of ± 0.5 °C in steady-state. In addition, the resulting Nusselt number (Nu) and the Fanning friction factor (f) data were compared with consolidate literature correlations showing an average discrepancy of 15% for both geometries and parameters. Therefore, the use of thermosyphons results in smaller, simpler and more precise test benches to be used for the evaluation of heat exchangers, as it provides the desired constant wall temperature conditions for the characterization of core geometries, substituting, with advantages, the use of large and expensive steam boilers.