Most gas furnaces use either a flame sensor or a thermocouple, sometimes 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.
Thermocouples vs. Flame Sensors
A thermocouple is typically used on a gas furnace with a standing pilot and is recognizable by its small, continuously burning flame that can be seen if you remove the access cover on the furnace's burner chamber. The top 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. A thermocouple is typically found in older furnaces, whereas new installations are now required to use electronic ignition furnaces, which use less energy since there is no pilot that burns gas constantly.
A thermocouple consists of a metal gas tube (usually made of copper), a probe that extends into the pilot flame, a bracket, and a wire that leads to the gas control unit. A bad thermocouple will generally show signs of damage on the tube, the wire, or the connecting nuts. If the thermocouple is bad, the furnace usually refuses to ignite (since the pilot light no longer burns).
Flame sensors are used in furnaces that use electronic ignition rather than a standing pilot light. These furnaces can use an intermittent pilot that ignites only when needed, or they can use a hot surface ignition system that uses heat to ignite the gas. These units have electronic igniters that light the gas, with a flame sensor that makes sure the burners have lit successfully. If there's a problem with ignition and the burners fail to light or go out, the flame sensor is designed to shut off the gas to the burners.
Like a thermocouple, the flame sensor has a metal probe and bracket, but it has no gas tube. Instead of a wire lead, it usually has a quick-disconnect wire fitting. The most common symptom of a bad flame sensor is a furnace that cycles on and off repeatedly every few seconds. A bad flame sensor may show visible signs of damage, such as a cracked ceramic insulator.
While both thermocouples and flame sensors can sometimes be cleaned in order to restore them to good operation, they are such inexpensive parts that most service technicians simply replace them if they suspect a problem.
Purchasing Replacement Parts
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 a 30-millivolts (mV) rating for standing-pilot furnaces. The length you see listed 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 specified 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.
Before You Begin
Before getting started, turn off the electrical power to the furnace by rotating the toggle switch mounted on the furnace to the off position. This switch is usually mounted on the furnace housing, but it may also be located on the wall near the furnace.
To turn off the gas to the furnace, use the valve handle located on the gas pipe running into the furnace. When the handle is perpendicular to the pipe, rather than parallel, the gas is off. Check that the knob on the gas valve is turned to off: There will likely be multiple settings, include off, pilot, and on.
Equipment / Tools
- Open-end wrenches
- Screwdriver or nut driver
- New thermocouple or flame sensor
How to Replace a Thermocouple
Open the Access Cover
Remove the furnace's access cover. If the furnace has been running, wait at least 30 minutes for the thermocouple to cool completely before continuing.
Remove the Old Thermocouple
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.
Install the New 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.
Connect the Gas
Thread the end fitting of the thermocouple 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 Power
Turn on the gas and electrical power to the furnace. Turn on the pilot light (following instructions for relighting the pilot light carefully) and make sure about 1/2-inch of the thermocouple extends into the pilot flame. Replace the furnace access cover and test the furnace operation by changing the thermostat settings to make sure the furnace turns on and off properly.
How to Replace an Electronic Flame Sensor
To turn off the gas to the furnace, use the valve handle located on the gas pipe running into the furnace. When the handle is perpendicular to the pipe, rather than parallel, the gas is off.Make sure the gas valve knob is turned to the off setting.
Locate the Flame Sensor
Remove the furnace's access cover. If the furnace has been running, wait at least 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, you'll need to call a professional furnace technician to make the repair.
Remove the Flame Sensor
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 screws with a nut driver or wrench.
Install the New Flame Sensor
Install the new flame sensor in the same position as the old part and secure it with the screws. Reconnect the electrical leads to the sensor and to the control box. Test the furnace to make sure it turns on and off properly.
Emergency Management and Communication. City of Chicago Utilities.