Modelling of buildings with natural or hybrid ventilation systems requires the coupling of a thermal and an air flow model because of the strong mutual impact of the thermal and the air flow behaviour. The newly developed tool TRNFlow is the complete integration of the multizone air flow and pollutant transport model COMIS [Dorer 2001] into the thermal multizone building module of the building and system simulation program TRNSYS [Klein 2000]. An internal solver algorithm using successive substitution with automatic adapted relaxation finds consistent solutions of the two models. The existing TRNSYS user interface for the building module has been updated for the easy and user friendly input of the additional air flow and pollutant transport model data.