what is a thermocouple for hot water heater?

Answer 1

It is a heat actuated magnetic device that controls the flow of natural gas to your heater. When they burn out the gas flow stops. They are not very expensive just a hassle to replace.

Answer 2

A thermocouple uses the micro current created by dissimilar metals to measure temperature.

Thermocouples are found on residential water heaters, primarily. They extend into the flame of a pilot light on one end, and screw into the temperature control on the other and allow it to make the water heater main burner fire when the thermostat senses that the water temperature has gone too low.