Heat pipe devices, for their typical working mechanisms, are particularly suitable for zero gravity applications, and have also been considered for applications in space satellites with nuclear propulsors, thanks to the absence of mobile systems for the coolant fluid circulation. The present work reports the results of experimental tests carried out on a heat pipe facility designed to investigate the thermal–hydraulic performance of a water heat pipe. The device layout, configuration and geometry, allow to simulate a heat pipe working utilizable in space applications and so under zero gravity conditions. The evaporating section, completely lined by wicks (sintered stainless steel), and nearly plane shaped, is housed in a cylindrical container. Two test series were performed with reference to different heater sizes (heated plate—wick contact surface size), obtaining for the evaporating heat flux q″ ev a value of 13 W·cm −2 in the 1st series and of 20 W·cm −2 in the 2nd one, using a smaller heater size. This result means a potential possibility to use more compact evaporators and to work at higher system operating pressure values. It was possible to establish the increase of the limit heat flux q″ evl versus the operating pressure in a range of 50–400 kPa. The condensate subcooling, due to the subcooler in the external channel, sensibly affects the boiling and the wick dispriming. In conclusion, it is necessary to point out that the obtained results are only preliminary and relative to a pilot test section. Further tests will be carried out with appropriate changes in the configuration, in the geometry and in the operating conditions.