A CPAP device includes a flow generator with a blower configured to pressurize the flow of respiratory gas. A flexible face seal is positioned at an outlet end of a blower discharge path. The flexible face seal includes an aperture and a lip curling inwardly from a perimeter of the flexible face seal. A tub configured to hold a body of water and humidify the pressurized flow of respiratory gas includes a heat conducting base plate and a side wall. The side wall has a substantially flat portion that surrounds an air inlet opening. A base supports both the flow generator and the tub and includes a floor with a heater plate. A laterally positioned guide prevents the tub from being removed from the base in a vertical direction and includes a pair of C-shaped channel portions with open sides that face each other.