Removable and/or replaceable humidifier

A respiratory treatment apparatus comprising:

• a positive airway pressure device to provide a source of flow of air at positive to ambient;
• a humidifier housing having a gas inlet to receive sealably the flow of air at positive pressure from said source in use and a gas outlet adapted to be connected to a gas conduit to deliver humidifier gas to a patient mask;
• a plate positioned within the humidifier housing;
• a water distribution member forming a water envelope for receiving liquid water, the water distribution member being fitted into the humidifier housing and including an upper wall and a lower wall between which the water envelope is formed, the upper wall being a semi-permeable material positioned below but spaced from the plate, and the lower wall being a heat conductive material;
• an electric heater in thermal contact with the lower wall and adapted to heat the liquid water to form water vapor;
• a gas passage layer between the gas inlet and the gas outlet, the gas passage layer configured to permit a gas flow to flow from the gas inlet to the gas outlet, the water distribution member being configured and arranged to deliver water vapor to the gas passage layer;
• wherein:
• the plate splits the gas passage layer into a humidification passage and a dry gas passage, the humidification passage being positioned between the upper wall of the water distribution member and the plate, the dry gas passage being positioned between the plate and a wall of the humidifier housing above the plate,
• an amount of the gas flow entering into the humidification passage and the dry gas passage is variable, and
• the semi-permeable upper wall of the water distribution member is configured to deliver the water vapor but not the liquid water to the humidification passage;
• and
• the water distribution member has a flange that is sandwiched between the humidifier housing and a base plate connected to the humidifier housing.