A breathing system (1) includes a gas source (10) connected to a patient interface (80) via a passageway (90). An in-line humidifier (50) is disposed along the passageway. The humidifier includes a humidification chamber (400) having a fluid inlet (420), a fluid outlet (430), and a reservoir (410). The humidifier includes a liquid trap (200) comprising a chamber (210), a fluid inlet (220) in communication with the liquid trap chamber, and a fluid outlet (230) in communication with the liquid trap chamber. The fluid outlet of the liquid trap is in fluid communication with the fluid inlet of the humidification chamber. The liquid trap is constructed and arranged to discourage liquid from back- flowing from the humidifier to the source of gas. For example, a spill height defined by the liquid trap's fluid inlet is elevated relative to a low point of an interior of the liquid trap regardless of an orientation of the humidifier.
