As a result of a 25 year history of model development and experimental validation, FLACS is established as a CFD-tool for simulating hydrocarbon gas deflagrations with reasonable precision. FLACS is widely used in petrochemical industry and elsewhere for explosion predictions for input to risk assessments and design load specifications. In recent years the focus on predicting hydrogen explosions have increased, and with the latest release the validation status for hydrogen deflagrations is considered good. For hydrogen explosions deflagration to detonation transition (DDT) may be a significant threat. An ongoing activity with support from the Norwegian Research Council is aiming at predicting with FLACS to what extent DDT may be expected. This article will discuss the validation status of FLACS-HYDROGEN, describe the new models to predict DDT, and compare predictions with relevant experiments.