A multiple purpose instantaneous gas water heater comprises a combination of a larger combustion capacity type first burner with a smaller combustion capacity type second burner. Each of the burners can be controlled by a proportional combustion control method and/or an intermittent combustion control method. It is possible to combine these functions within a microcomputer system so as to use each or both burners selectively, or both together, so that it is possible to select water from a wide range of hot water temperatures or to select a target temperature.