Quck answer
Replacing a gas furnace thermocouple is a relatively simple process that can be done by following a few steps. First, turn off the gas supply to the furnace and disconnect the old thermocouple from the gas valve. Next, remove the burner assembly to access the thermocouple. Unscrew the old thermocouple from the burner assembly and replace it with a new one. Reassemble the burner assembly and reconnect the thermocouple to the gas valve. Finally, turn on the gas supply and test the furnace to ensure it is working properly. It is important to follow safety precautions and consult the furnace’s manual for specific instructions.
A thermocouple is a safety device that is found in older furnaces and boilers. It prevents gas from flowing to the burners if there is no pilot flame present to ignite it. This ensures that your home is safe from natural gas leaks. The thermocouple is only present in older furnaces that have a standing pilot light, which ignites the flow of gas to the burners. However, newer furnaces and boilers use electronic “direct spark” or hot surface igniter systems which are more efficient.
If your furnace is less than 20 years old, it is unlikely that it has a standing pilot or a thermocouple. Boilers, on the other hand, may still have standing pilots and thermocouples.
How Does a Thermocouple Work?
The thermocouple is located in the burner chamber of the furnace. It consists of a long, thin wire or tube with a sensor rod on one end. The sensor rod fits into a bracket that extends into the pilot flame. The opposite end of the thermocouple is connected to the gas valve. As long as the sensor rod remains heated by the pilot flame, it keeps a faint electrical current running to the gas valve, allowing the flow of gas to the burners for ignition. When the pilot flame goes out, the electrical signal is cut off, and the gas valve shuts off the flow of gas.
Signs of a Faulty Thermocouple
If your furnace stops producing heat, it may be because the standing pilot is out. If your pilot flame goes out and refuses to stay lit when you try to relight it, it is likely that the thermocouple needs to be replaced.
Before replacing the thermocouple, it is recommended to clean the tip of the thermocouple and ensure that it is properly positioned in the pilot gas jet. To clean the thermocouple, use emery cloth or steel wool to remove any corrosion from the sensor tip. Make sure to brush away or vacuum out any sooty residue removed from the thermocouple sensor. At the same time, check that the tip is in the center of the small flame emerging from the pilot gas jet. If these steps do not solve the issue, it is time to purchase and install a new thermocouple.
When purchasing a new thermocouple, make sure to buy one that matches the length of your old thermocouple. Universal aftermarket thermocouples are available and usually work well. You can also order a thermocouple that meets the original manufacturer’s specifications from an online parts retailer, but you need to specify the manufacturer and model number of your furnace to ensure you are getting the correct part.
Things You’ll Need
-
New thermocouple
-
Nut driver set
-
Screwdriver
-
Flashlight or work light
-
Adjustable wrench or open-end wrench set
Step 1: Shut Off the Furnace
First, turn off the switch that powers the furnace or boiler. This is usually an ordinary wall switch mounted on the side of the furnace, or it may be mounted on a nearby wall. If there is no switch present, turn off the furnace’s circuit breaker at the main service panel (breaker box).
Step 2: Shut Off the Gas
Locate the shutoff valve on the gas supply pipe leading to the furnace and turn the valve so its handle is perpendicular to the pipe. This will turn off the gas supply. Also, turn the knob on the furnace’s gas control valve to the “off” position.
Step 3: Open the Access Panel
Use a nut driver or screwdriver to remove the mounting screws on the upper access panel of the furnace and remove the panel. Use a flashlight or work light to locate the thermocouple within the compartment. The thermocouple consists of a wire or tube that ends in a thicker sensor rod mounted to a bracket adjacent to a small pilot gas jet, immediately next to the furnace’s main burners. The opposite end of the thermocouple connects to a fitting on the gas control valve.
If desired, you can first clean the sensor rod on the old thermocouple, adjust its position, and then try to relight the pilot. In some cases, this may solve the problem without replacing the thermocouple.
Step 4: Loosening and Removing the Thermocouple
To remove the thermocouple, first, loosen the nuts that connect the wire/tube end to the gas control valve and the nut that holds the sensor rod to the bracket next to the pilot gas jet. In some instances, the sensor rod end is clipped in place. Once loose, slide the sensor out through the bracket and then carefully remove the entire thermocouple.
Step 5: Installing the New Thermocouple
Begin by uncoiling the new thermocouple and threading the rod end into the bracket near the pilot gas jet and the wire end into the fitting on the gas control valve. Route the wire within the compartment in the same way the old thermocouple was. Then, use an adjustable wrench to tighten the nuts and ensure that the tip of the rod is in the flame of the pilot gas jet.
Step 6: Turning On the Furnace and Gas
Start by opening the shutoff valve on the gas supply pipe, ensuring that the handle is parallel to the pipe, then turn on the power switch for the furnace.
Step 7: Relighting the Pilot
Turn the control knob on the gas valve to the “pilot” setting, push in the knob, and hold it in. If there is no “ignite” button present, hold a match or lighter to the pilot tube jet or push the “ignite” button. Keep holding the knob in for about 20 seconds after the pilot flame ignites. After that, release the knob and ensure that the flame continues to burn. If not, repeat the lighting procedure.
Step 8: Replacing the Access Panel
Attach the access panel on the burner chamber and test the furnace by adjusting the thermostat up and down several times to ensure that the furnace starts and stops correctly.