This paper referrer to the existence of the variety modes and the 'chaos' in building ventilation system, and provides some instances of such varieties and the chaos in forced ceiling chamber smoke exhaust system in building fire. The numerical calculations are executed by the Newton-Raphson method for the ventilation network models. The ventilation variety up to five modes and the chaos characteristics are obtained as the natural result of the non-linear system and illustrated in figures and maps. We inquire further into the dangerous back flow at the smoke vent which may be caused by certain conditions of the heat supply rate and the geometry of ceiling space.