Abstract: A hydrogen fuel cell water knockout device for an exhaust system of a heavy duty hybrid hydrogen fuel cell transit bus includes an expansion chamber housing forming an expansion chamber, the expansion chamber including a substantially vertical exhaust section and an exhaust flow path there through with a vertically lowest point in the exhaust flow path; a drain; and a condenser disposed at the vertically lowest point in exhaust flow path of the substantially vertical exhaust section in the expansion chamber so that the condenser condenses water from H2O exhaust without reintroduction of the water into the exhaust flow path and the water drains from the hydrogen fuel cell water knockout device through the drain.