A flow switch for a CPAP breathing system uses a Coanda effect to switch a flow of CPAP gases away from a patient outlet port (3) during patient exhalation. The flow switch has an inlet passage (2) leading to a constricted nozzle (5). A chamber (6) immediately downstream from the constricted nozzle (5) has a patient outlet passage (3) and a vent passage (4). A first wall (7) extends from the nozzle to patient outlet passage (3). A second wall (8) extends from the nozzle (5) to the vent passage (4). Separation between the wall grows moving away from the nozzle (5). Provision is made for influencing flow from the nozzle toward the first wall (7) during patient inhalation. The flow attaches to the first wall (7) during patient inhalation, and to the second wall (8) during patient exhalation.
