How to Replace a Gas Furnace Thermocouple

In older furnaces and boilers, the thermocouple is a simple safety device that prevents gas from flowing to the burners unless a pilot flame is present to ignite it. This ensures that natural gas won't fill your home if there's a problem with the furnace. This simple but important part is found only on older furnaces and boilers that have a standing pilot light, which is a tiny flame that remains burning all the time, waiting to ignite the flow of gas to the furnace's burners when the thermostat sends the signal to begin heating.

Thermocouples are common in furnaces manufactured before 2000, but later furnaces and boilers generally use electronic "direct spark" or hot surface igniter systems. Because there is no standing pilot that burns gas constantly, these newer furnaces are considerably more efficient than older designs.

With the average forced air furnace lasting about 20 years or so, furnaces with standing pilots and thermocouples are gradually phasing out in favor of newer designs. If your furnace is less than 20 years old, there's a good chance it doesn't have a thermocouple or standing pilot at all. Boilers, however, sometimes have a longer life span, and there are plenty of hot water/steam heating systems with standing pilots and thermocouples still in operation.

The thermocouple is a device that converts heat to an electrical charge, a process that naturally occurs when heat passes through two dissimilar metals in contact with one another. You will find the thermocouple in the burner chamber of your furnace. It consists of a long, thin wire or tube with a rod-shaped flame sensor on one end. The sensor rod fits into a bracket positioned so its tip extends into the tiny pilot flame at the end of the pilot gas jet. At the opposite end of the thermocouple, the wire or tube is connected to a fitting on the main 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. This signal tells the gas valve it is safe to open and allow the flow of gas to the burners, where it will be ignited by the pilot flame.

Should the pilot flame go out, no electrical signal is sent to the gas valve, and no gas is allowed to flow. Thus, the thermocouple is a critical safety device, preventing gas from flowing unless the pilot flame is present to ignite it.

When a gas furnace stops producing heat, it may be because the standing pilot is out. It's not uncommon for a standing pilot to go out, but if your pilot flame refuses to stay lit when you try to relight it, there's a good chance that the thermocouple needs to be replaced.

Before replacing the thermocouple, it's worth trying to clean the tip of the thermocouple and make sure the tip is properly placed in the pilot gas jet. Cleaning a thermocouple is a simple matter of using emery cloth or steel wool to remove any corrosion from the sensor tip of the thermocouple. Make sure to brush away or vacuum out any sooty residue removed from the thermocouple sensor. At the same time, check the adjustment of the tip within the bracket. The thermocouple won't operate correctly unless the tip is in the center of the small flame emerging from the pilot gas jet.

If these fixes don't solve the problem, then it's time to buy and install a new thermocouple.

Thermocouples come in many sizes and lengths. There are universal aftermarket thermocouples widely available that will usually work just fine. The thermocouples for furnaces usually produce 30 millivolt (mV) current but make sure to buy one that's the same length as your old thermocouple. Lengths of 18, 24, 30 and 36 inches are standard.

You can also order a thermocouple that meets the original manufacturer's specifications from an online parts retailer, such as Repair Clinic or Appliance Parts Pro. You will need to specify the manufacturer and model number of your furnace to ensure you're getting the right part.