The residential fuel cell system was modeled as a box-shaped chamber with vent openings,filled with various components such as reformer, desulfurizer, fuel cel stack and humidifier. When the ventopenings are 1%2of the total surface and hydrogen leakage 1%2 hydrogen concentration is around 0.1%2s increased from 0.3%2to 0.7%2in the upper region of the system after 200 seconds. When the vent openingsare 1%2of the total surface and hydrogen leakage 1%2 3%2 5%2 the steady state result of CFD, 5%2ofhydrogen leakage is reached the lowest ignition limit in the system. When the vent openings are 2%2ofthe total surface and hydrogen leakage 1%2 hydrogen concentration is increased in the bottom of the systemfor 60 seconds. After 250 seconds, hydrogen concentration is reached the steady state in the system. Asthe vent opening of the total surface increased from 1%2to 2%2 averaged hydrogen mole fraction is under1%2in the system, however, upper regions of the system from the hydrogen leakage points are shown over1%2of hydrogen mole fraction. F , 4 ( , , , ) 30%2 , . 1%2 1%2 , , 50 , 200 , . 1%2 1%2 3%2 5%2 , 5%2 (4%2 ) . 2%2 1%2 , , 60 , 250 . 2%2 1%2 3%2 5%2 , 5%2 (1%2 ) , .