There are two types of water heater thermocouples: standing pilot and electronic ignition. The type of ignition system used in the hot water heater determines which thermocouple is required.
Thermocouples are used in gas hot water tanks to control gas flow. In many cases, a malfunctioning thermocouple is the reason why a gas water heater fails to ignite or stay lit. In a hot water tank with a pilot light, the thermocouple and pilot are attached to the burner assembly; in an electronic ignition system, the pilot and thermocouple are grouped with an electronic igniter. In both systems, the thermocouple is attached to the gas control valve on the outside of the tank.