Combined heat, air, moisture and pollutant simulations (CHAMPS) at the building system level are essential for improving energy efficiency and indoor environmental quality. This paper discusses the technical challenges and possible solutions to the problem of coupling an envelope model (CHAMSBES) with a multizone/network model for inter-zonal air and pollutant transport. A representative multizone solver was written, which solves the coupled heat, air, moisture and pollutant transport equations. Several different methods for coupling this representative CHAMPS-multizone code with the CHAMPS-BES code were investigated. The implementation details of these solutions are presented and evaluated with respect to numerical performance, stability and flexibility. A recommended implementation is shown, which allows flexible coupling of different independent simulation codes, while providing an acceptable numerical accuracy.