A system, apparatus and methods are provided for supplying gases to a user. The supply includes a sub-therapeutic mode and a pressure support mode for delivering therapy to a user. A flow diversion device or valve switches from a first mode corresponding with the sub-therapeutic mode of the system to a second mode corresponding with the pressure support mode of the system. In the first mode, the valve opens a larger flow path between the interior of the user interface and ambient air than in the second mode.