A humidifier has a base unit with an engagement face that is configured to interface with a flow generator. The humidifier also has a tank configured to be removably received by the base unit and hold a volume of liquid. The tank has a side wall with an air inlet. The humidifier further has an air flow passage configured to receive an air connector of the flow generator at the engagement face of the base unit. The air flow passage is axially offset from a tank inlet. In addition, a cross-section of the airflow passage inlet and a cross-section of the tank air inlet are substantially perpendicular to a horizontal plane.