Abstract: A control valve system for a gas-fired, instantaneous water heater is disclosed. The valve has a "snap" action constructed to inhibit the supply to the burner of insufficient amounts of gas to ensure reliable ignition. In one example, a flexible diaphragm has opposite sides connected via respective passages to receive pressures upstream and downstream of a restriction in a water flow path. The diaphragm opens a gas valve in response to a predetermined flow of water along the path. In order to provide the "snap" action, a further passage is provided that links the two passages coupled to the diaphragm and contains a ball that can block the further passage only when the water flow is sufficient to open the gas valve sufficiently for an ignitable quantity of gas to flow to the burner.