Variable refrigerant flow (VRF) air conditioning system has become attractive due to better energy performances than traditional air conditioning systems. However, the shortcoming of no outdoor air (OA) intake has not been solved thoroughly. A new VRF and outdoor air processing unit combined air conditioning system is proposed and simulated. The first obstacle is that there is no well-known simulation tool for VRF unit in heating mode. A VRF model of condenser-number independence is developed and validated first. The combined system is modeled by integrating the individual sub-system or component models into a complete system. The average error of the developed model to predict heating capacity, input power and COP are 7.87%, 12.45% and 6.19% respectively. Finally the combined system is simulated under conditions of the same and different set-points of the air conditioning zones. The combined system could maintain all the zones at their specific set-points within small errors no matter the set-points are the same or different. Moreover, indoor air quality can be ensured. The demand of one air conditioning system possessing independent units serving separate zones in the same building could be met by the combined system. (C) 2013 Elsevier B.V. All rights reserved.