This paper reports on the advancement in the study of thermal-hydraulic dynamic instabilities with reference to the helical-coiled tube geometry. A full-scale open-loop experimental facility simulating a helically coiled steam generator was built and operated at SIET labs in Piacenza (Italy). The facility comprises two helical tubes (1 m coil diameter, 32 m length, 8 m height), connected via lower and upper headers. Nearly 100 flow instability threshold conditions were identified, in a test matrix of pressures (80 bar, 40 bar, 20 bar), mass fluxes (600 kg/m2 s, 400 kg/m2 s, 200 kg/m2 s), inlet subcooling (from −30% up to ∼0), and inlet throttling (four different entrance resistance conditions). The long test section feature and the helical-coiled tube geometry render the present facility a quite unique test case in the outline of two-phase flow instability experimental studies. Parametric effects of the operating pressure, flow rate, inlet subcooling and inlet throttling on the threshold power are discussed. The period of oscillations is also discussed. Superimposition of Density Wave Oscillations (DWOs) with Ledinegg flow excursions is finally described. Theoretical modelling of DWO occurrence in helical pipes was addressed by means of a lumped parameter analytical model, which was exploited to highlight some peculiarities of DWO phenomena and respective stability boundary with respect to classical straight geometry. In the end, numerical simulation results with RELAP5/MOD3.3 code were compared.