A backup water heater for installation downstream of a water outlet of a thermosiphon solar water heater includes a corrosion resistant tank having a fin heat exchanger. A burner is supplied with gas from a gas feed valve system that includes at least one electrovalve connected to a cast body configured. A bimetallic temperature detector is disposed to detect the temperature of water in the tank, and a flow detector is disposed so as to activated by water flow through the tank. A first spark plug ignites the burner, and a second spark plug positioned to detect current changes in the first spark plug generated by ionization effect after the burner is ignited. An electronic controller is in operative communication with the various components to send a current to the first spark plug upon detection of a low setpoint temperature from the temperature detector and a detected setpoint flow from the flow detector. The electronic controller causes gradual opening of the electrovalve to supply gas to the burner to ignite the burner with the first spark plug. The electronic controller detects the change of current from the second spark plug and generates a visual display of normal operation, or closes the electrovalve and terminates operation of the water heater if the change of current is not detected within a defined time.