Based on the thermal and airflow network model with simple but perfect generic formulations and stable solving methods, a computer program NETS for the practical simulation of coupled building heat, gas and air transfer system has been developed. NETS can simulate the feedback or the schedule control on models' structural changes and various driving condition changes. A pre-processing system called NETSGEN and a post-processing system called NETSOUT have been also developed. NETSGEN enables designers to freely construct the thermal and airflow network model on energy saving new ideas with an ease of simply drawing a picture. NETSOUT graphically displays the computed results. NETS is versatile and especially suitable for use in research, development and design of passive and low energy architecture.