Abstract: An apparatus accurately controls output water temperature of an instantaneous water heater to maintain a desired outlet temperature. A first electronic sensing device senses a flow of water through the heater, and an ignition sequence commences when flow has been detected. A second electronic sensing device senses an outlet water temperature of the heater, and a third electronic sensing device senses a midway water temperature of the heater. The burner output or outputs are controlled to maintain the desired outlet temperature, based on changes in water flow and incoming water temperature. With multiple burners, the burner outputs are staged to accurately control the output water temperature of the instantaneous water heater to maintain the desired outlet temperature.