An evaporator defines an evaporator volume, and boils the liquid inside evaporator volume. A mixing chamber is in communication with the evaporator volume and mixes the vapor or moisture from the liquid in the evaporator with the breathing gas of the patient. The mixing chamber is a separate structure than the evaporator. The mixing chamber and the evaporator are connected by a first connection that is repetitively connectable and disconnectable without significant destruction of a respective connection. A heater is arranged outside of the evaporator and in a heat conducting connection with the evaporator. The heater is a separate structure than the evaporator. The evaporator and the heater are connected by a second connection that is repetitively connectable and disconnectable. A liquid storage tank is in communication with the evaporator and supplies a liquid to the evaporator through a flow channel. The liquid storage tank is larger than the evaporator. The flow channel is formed of a material and has a size to thermally insulate the evaporator from the liquid storage tank.