Carbon monoxide (CO) poisoning accidents caused by the gas-burning water heaters, due to poor ventilation, in redsiential buildings is one of the common mishaps in Taiwan. Most previous studies on this problem employed Computational Fluid Dynamics (CFD) models to simulate the dispersion of CO inside the buildings. Nonetheless, their results did not consider the effects of external wind speed and direction to the transport of CO inside the buildings. This study incorporates a natural ventilation model and a gas transport model to predict the ventilation rate and CO concentration in partitioned buildings. The model predictions are validated by the measured concentration, by a tracer gas technique, in wind tunnel experiments. The verified transport model is used to evaluate the influences of initial concentration, interior volume, external wind speed and direction on the concentration variation of CO in a full-scale two-room building. The results reveal that the ratio of the maximum concentration in the adjacent room to the initial concentration of CO in the room (balcony) with the source was about 50~62%. The ventilation rate increases as the wind speed and the opening area increases, this sequentially reduces the dispersion time of CO in the building when the wind direction is normal to the windward opening. The ventilation rate drops significantly and the CO concentration remains very high for a long period of time, and mechanical ventilation is needed to disperse the CO inside the building.