Abstract: An instantaneous demand gas fired hot water heater apparatus having the water temperature sensing thermostat disposed in the upper hot water outlet header with the inlet flow of unheated water directed to flow in a preferred flow path across the temperature sensing element. The temperature sensing element location in the upper header of the heater provides an extremely sensitive and efficient burner control. Upon initial output demand of hot water the lower temperature of the unheated water is immediately sensed by the temperature sensing element to immediately commence burner operation. Upon termination of heater water demand, the heated water temperature is rapidly sensed by the temperature sensing element to promptly terminate the supply of the gas to the burner and minimize unnecessary heating of the water.