A. O. Smith gas water heaters heat water by burning either natural gas or LP (liquid propane). All models have a thermostat/gas control to set the desired temperature. When a hot water fixture is opened, hot water flows out of the tank and through hot water lines in the home and the tank is refilled with cold water. The thermostat monitors the water temperature and initiates reheating when needed. Standard gas models have a standing pilot light, and when the thermostat calls for operation gas flows to the burner where it is lit by the pilot light. High-efficiency models utilize an electronic gas valve and use a spark or hot surface ignitor so a standing pilot is not required. Standard gas models have a standing pilot light, and when the thermostat calls for operation gas flows to the burner where it is lit by the pilot light. When the desired water temperature is reached in the tank, the main gas valve shuts off and the heating process stops.