FHTS is a major extension to the existing ESATAN thermal analyser. It is capable of carrying out one dimensional steady state or transient analysis, solving for pressure, temperature and mass flow rate, for single or two phase loops.This paper begins by giving an overview of the capabilities of FHTS and outlines the stringent quality assurance standards adhered to during its development.The coding standards ensure software verification during its development via module testing and the development of a test suite. Further verification of both the single phase and two phase capability has already been carried out by comparison against Retran, a well established package used throughout the Nuclear Industry. An outline of the results from this work and conclusions drawn is given. Although limited in scope the work proved extremely valuable.A first step in validating FHTS has been carried out by ESA in 1990 for the single phase package by comparing experimental data against simulated results. To achieve this goal, the hybrid radiator under development in the frame of the Columbus program has been used. The experimental data was taken from the final thermal vacuum test which took place at Estec in March 1990. The validation work proved conclusive.An experimental two phase test loop has been developed and results from this are expected early 1991. Within a coming upgrade validation of the two phase routines is to be carried out using these results as a datum.Pressure from industry prompted a review of the numerical treatment of the algorithms within FHTS with a view of improving the speed of the software. Excellent results from this first pass of efficiency improvements have been achieved and a resume of this is given.The first major upgrade of the software has just begun and this paper concludes by giving a detailed account of the improvements to be made and the new facilities to be provided.