The Columbus Orbital Facility (COF) is being developed as the European Laboratory contribution to the International Space Station (ISS) program.The COF Active Thermal Control System (ATCS) is based on a single phase water loop for the collection of waste heat from payloads and subsystems and the rejection to the ISS Heat Exchangers (HXs).During 1993 the budget constraints and the shifting to November 2002 of the European Laboratory launch led to an incremental qualification approach for the COF ATCS water loop: to qualify the ATCS water loop by analysis using mathematical models correlated throughout a series of testing.The water loop testing has been developed through different incremental steps whose results were correlated by means of ESATAN-FHTS models. The water loop setup testing was upgraded, step by step, by using commercial and flight representative items on the basis of the equipment development status.The First Step was aimed at evaluating the hydraulic modelling of the components typically used in a water loop and creating a database for the build-up of FHTS thermal-hydraulic models. The Second Step had as main object to verify the behaviour of the mathematical tool to represent closed loop controls. Flight design representative components were inserted in the Third Step loop to prove the performance compatibility of the single component with the whole subsystem. During the Fourth Step the ATCS Control S/W was implemented, the H/W to S/W interaction tested and, finally, the FHTS model correlated.This Paper deals with all the activities performed from the test article definition to the mathematical correlation, with particular attention to the external interfaces simulation, hardware to control software interfaces and model numerical convergence.