Gas Furnace Flame Sensor vs Thermocouple: What’s the Difference?

Most gas furnaces have either a thermocouple or a flame sensor. They are safety devices that work automatically when a gas furnace is running. If your home has a gas furnace, it will likely feature either a flame sensor or a thermocouple. What’s the difference between these two furnace-related safety devices exactly?

What Is a Thermocouple?

A thermocouple is a safety device that’s designed to detect – as well as respond to – the presence of a flame with a pilot light. A pilot light, of course, is an ignition system. It creates a spark to ignite the natural gas released by the furnace’s valves. Around the top of the pilot light is a thermocouple.

The thermocouple will detect the presence of a flame with the pilot light. As long as the pilot light is lit, the thermocouple will allow the furnace’s valves to remain open. If the pilot light is out, conversely, the thermocouple will close the valves.

What Is a Flame Sensor?

A flame sensor is a similar type of safety device in gas furnaces. It controls the furnace’s valves by detecting the presence of a flame. With a flame, the flame sensor will allow the furnace’s valve to remain open. Without a flame, it will force the furnace’s valves to shut.

Thermocouples and flame sensors are designed to control gas valves. Gas furnaces have valves that release natural gas. Some gas furnaces have a thermocouple to control these valves, whereas others have a flame sensor. Thermocouples and flame sensors are responsible for opening and closing these valves depending on whether or not a flame is present.

Differences Between Thermocouples and Flame Sensors?

Gas furnaces require either a flame sensor or thermocouple to prevent excess and unburned gas from being released. The main difference between these two safety devices is that thermocouples are used with pilot lights, whereas flame sensors are used with electronic ignition systems.

In the past, all gas furnaces had a pilot light. Pilot lights are still found on some gas furnaces, but most modern gas furnaces now have an electronic ignition system. If you have a new gas furnaces, chances are it features an electronic ignition system.

Electronic ignition systems still produce a flame. As their name suggests, though, electronic ignition systems use electricity. The bottom line is that old gas furnaces with a pilot light use a thermocouple, whereas newer gas furnaces with an electronic ignition system use a flame sensor.