A system for non-invasive ventilation (300,400,900,1100,1400) comprising: a gas source conduit (235,240) adapted to be fluidly coupled at a first end to a gas source and comprising at a second end a bifurcation having a first branch and a second branch; a primary flow path conduit adapted to be coupled to the first branch of the bifurcation as part of a primary flow path (260,1460); a flushing flow path conduit adapted to be coupled to the second branch of the bifurcation as part of a flushing flow path (250,1450) having a higher resistance to gas flow than primary flow path; a patient interface comprising a breathing chamber and a nasal flow delivery part, constructed such that the breathing chamber is in the primary flow path (260,1460) and the nasal flow delivery part is in the flushing flow path (250,1450); a control assembly (220,320,820,1420) coupled or adapted to be coupled to the primary flow path (260,1460), the control assembly (220,320,820,1420) comprising a movable member (1121,1421,1721,2021,2024) movable between a first position in which the movable member increases the resistance to gas flow through the primary flow path (260,1460) and a second position in which the movable member does not increase the resistance to gas flow through the primary flow path (260,1460), the movable member (1121,1421,1721,2021,2024) configured to move between the first position and the second position in response to pressure changes within the breathing chamber of the patient interface; and wherein the movable member (1121,1421,1721,2021,2024) is configured to move to the first position when gas pressure within the breathing chamber of the patient interface is greater than about a gas source pressure and move to the second position when gas pressure within the breathing chamber is less than or equal to about a gas source pressure. a gas source conduit (235,240) adapted to be fluidly coupled at a first end to a gas source and comprising at a second end a bifurcation having a first branch and a second branch; a primary flow path conduit adapted to be coupled to the first branch of the bifurcation as part of a primary flow path (260,1460); a flushing flow path conduit adapted to be coupled to the second branch of the bifurcation as part of a flushing flow path (250,1450) having a higher resistance to gas flow than primary flow path; a patient interface comprising a breathing chamber and a nasal flow delivery part, constructed such that the breathing chamber is in the primary flow path (260,1460) and the nasal flow delivery part is in the flushing flow path (250,1450); a control assembly (220,320,820,1420) coupled or adapted to be coupled to the primary flow path (260,1460), the control assembly (220,320,820,1420) comprising a movable member (1121,1421,1721,2021,2024) movable between a first position in which the movable member increases the resistance to gas flow through the primary flow path (260,1460) and a second position in which the movable member does not increase the resistance to gas flow through the primary flow path (260,1460), the movable member (1121,1421,1721,2021,2024) configured to move between the first position and the second position in response to pressure changes within the breathing chamber of the patient interface; and wherein the movable member (1121,1421,1721,2021,2024) is configured to move to the first position when gas pressure within the breathing chamber of the patient interface is greater than about a gas source pressure and move to the second position when gas pressure within the breathing chamber is less than or equal to about a gas source pressure.