Difference Between a Thermocouple and a Flame Sensor
A thermocouple is used on a standing pilot furnace (with a pilot light). An electronic ignition furnace (intermittent pilot or hot surface ignition) uses a flame sensor. The flame sensor and thermocouple both perform similar functions in that they both control the flow of gas from the gas valve.
The flame sensing rod can be straight or “L” shaped. In both cases the thermocouple or flame sensing rod are generally part of a pilot burner assembly and both connect to the gas valve or to the electronic control box.