A respiratory humidifier has a base with a thermally controlled heater therein effective to heat a first transfer surface on the top of the base. Removably pressed against the first transfer surface is a second transfer surface forming the bottom of a container into which liquid is admitted, close to the bottom, by a float valve to establish a liquid level. Gas is admitted to the container below the liquid level through an inlet tube and is released from the container above the liquid level through an outlet tube. A porous pad in the container is in the gas flow path between the inlet tube and the outlet tube to provide extended, wettable surface. A float blocks the entrance to the outlet tube when the liquid level rises unduly and a normally closed shunt passage between the inlet tube and the outlet tube opens by differential pressure when the entrance to the outlet tube is blocked.