A humidifier for use with a pressure support system. The humidifier includes a body having an inlet, a fluid holding chamber, and an outlet. The inlet is positioned upstream and in fluid communication with the fluid holding chamber. The outlet is positioned downstream of and in fluid communication with the fluid holding chamber. A back-flow preventing valve is positioned upstream of the fluid chamber. The back-flow preventing valve is movable between an open position, in which the inlet is unblocked, and a closed position in which the inlet is blocked. In the closed position, the back-flow preventing valve prevents fluid, fluid vapor, or both from entering the pressure support via the inlet to the humidifier.