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.
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