Most gas furnaces use either a flame sensor or a thermocouple, often referred to as a "thermal coupler." Both are safety devices that sense the presence of a flame and control the flow of gas to the appliance accordingly. If no flame is present, the sensor stops or prevents the flow of gas from the gas valve, thus preventing the dangerous situation of gas flowing into the appliance when there is no flame to burn it. Flame sensors and thermocouples are simple parts that wear out and fail over time, and most are easy to replace.
Thermocouple vs. a Flame Sensor
A thermocouple, recognizable by its small, continuously burning flame, is typically used on a gas furnace with a standing pilot. The tip of the thermocouple is positioned in the flame, keeping the tip heated at all times. If the pilot flame goes out, the tip cools, and the thermocouple automatically shuts off the furnace's gas valve.
Flame sensors are used in furnaces with electronic ignition, rather than a standing pilot light, and might include an intermittent pilot or a hot surface ignition type. These units have electronic igniters that light the gas for the burners, while the flame sensor makes sure the burners have lighted successfully. If there's a problem with ignition and the burners fail to light, the flame sensor shuts off the gas to the burners.
Purchasing a Replacement Thermocouple or Flame Sensor
When purchasing a replacement thermocouple or flame sensor, it's essential to make sure that it's compatible with your particular furnace model. Honeywell, White Rodgers, and other manufacturers make universal replacement thermocouples, usually with 30 millivolt (mV) ratings for standing-pilot furnaces. The length you see on the package, such as 24 or 30 inches, is the length of the thermocouple's lead, which is the flexible metal wire between the fitting end that attaches to the gas valve, and the thermocouple tip that sits in the pilot flame inside the furnace.
Electronic flame sensors are much less universal, and you must find the exact part for your furnace model. Shop online through appliance parts dealers and compare their prices to those at local distributors. If you need a part in a hurry, find a local dealer who has the part in stock.
How to Replace a Thermocouple
These are the basic steps for replacing a furnace thermocouple. Always follow the manufacturer's instructions specific to your model:
- Shut off the gas and electrical power to the furnace, and remove the furnace's access cover. If the furnace has been running, wait 30 minutes for the thermocouple to cool completely.
- Using an open-end wrench, unscrew the nut that holds the thermocouple end fitting into the gas control valve.
- Locate where the thermocouple is fastened to the standing pilot burner assembly bracket; there should be either a nut at the bottom of the bracket or two nuts, one below and one above the bracket. Loosen the nut(s) and remove the thermocouple.
- Straighten the lead on the new thermocouple, and shape it to resemble the old thermocouple. This doesn't have to be precise; it just helps to start with a similar shape.
- Connect the new thermocouple to the pilot burner assembly bracket, tightening the nut(s) until it's just snug. Be careful not to overtighten.
- Thread the end fitting into the gas control valve and tighten it by hand. Then, use the open-end wrench to tighten it about a quarter-turn more; again, do not overtighten.
- Turn on the gas and electrical power to the furnace.
- Turn on the pilot light and make sure about 1/2 inch of the thermocouple is in the pilot flame.
- Replace the furnace access cover.
How to Replace an Electronic Flame Sensor
Follow these basic steps for replacing a flame sensor. Always review the manufacturer's instructions specific to your model:
- Shut off the gas and electrical power to the furnace. Remove the furnace's access cover. If the furnace has been running, wait 30 minutes for the flame sensor to cool completely.
- Confirm that the flame sensor is removable; if so, it should be fastened to the burner assembly with one or more screws. If the sensor is integral to the gas ignition system, call a professional furnace technician for help.
- Disconnect the electrical wire leads to the sensor. Disconnect the other end of the leads at the control box.
- Remove the flame sensor by loosening the fastening screw(s) with a nut driver or wrench.
- Install the new flame sensor in the same position as the old part and secure it with the screw(s).
- Reconnect the electrical leads to the sensor and to the control box.
- Replace the furnace access cover.