Gas furnaces use either a thermocouple or a flame sensor. Both are safety devices used to sense the flame and control the gas that flows into the appliance. Without a flame, the flame sensor prevents the flow of gas at the valve, preventing a situation where gas flows into the appliance without a flame to burn it. For this reason, you should practice regular flame sensor cleaning.

Thermocouples and flame sensors are simple devices, however, they do wear out over time. Once a flame sensor begins to malfunction, you should look to replace it with a new one. Flame sensor cleaning is as important as any other home appliance maintenance routine.

Flame Sensor Vs. Thermocouple

The thermocouple is smaller in size and used with continuously burning flames with standing pilots. It is positioned within the flame, which means the tips are continuously heated whenever there is a flame. Once the flame goes out, the thermocouple’s tip cools, signaling the thermocouple to block the passage of gas past the valve.

On the other hand, flame sensors are typically used in electrically ignited flames and not standing pilot flames, as is the case with thermocouples. Flame sensors might sometimes include a hot surface ignition or intermittent pilot model. The unit has an electric igniter that lights up the gas, while the flame sensor ascertains that the burner is ignited successfully. Whenever there is an issue with the ignition, and the gas burner fails to ignite, the flame sensor will automatically block the gas coming to the burner.

Shopping and Replacing a Flame Sensor or Thermocouple

When shopping for a new flame sensor or thermocouple to replace the old malfunctioning one, you should make sure it is compatible with the furnace model in your house. Most thermocouple manufacturers offer universal models that will work efficiently with any standard furnace model.

You should check the length of the thermocouple or flame sensor on its packaging – it’s often indicated in inches. This is the length of the lead, which happens to be the flexible metallic wire between the thermocouple tip sitting within the pilot flame and the end attached to the gas valve.

On the other hand, flame sensors are less universal, meaning you will have to find a perfect match for your kitchen’s flame model. With so many options out there, you can compare the prices to find a high-quality flame sensor that is also 100 percent compatible with your flame model at a cost-friendly price. You can also compare the flame sensor sold online with those found at your local distribution store. If you urgently need one, you can look for a dealer who has the flame sensors in-store.

How to Replace the Thermocouple?

It is not rocket science replacing a malfunctioning thermocouple. Although it’s highly recommended that you hire professionals for the job, you can replace the thermocouple yourself by following basic steps.

Pro Tip: Always follow the manufacturer’s instructions and guidelines that are specific to your flame model. Below are the steps you should follow.

  1. Turn off the electrical power and gas supply to the furnace, followed by removing the access cover. If the furnace has been in use, you can give it about 30 minutes for the attached thermocouple to cool.
  2. Unscrew the nuts holding the thermocouple’s end fitting against the gas control valves using an open-end wrench.
  3. Find the location where your thermocouple is attached to the standing pilot assembly bracket. It’s often fastened with one or two nuts either at the bottom or one above and one below the bracket. Loosen these nuts to remove the thermocouple.
  4. You should then straighten your new thermocouple to make sure it resembles the old one. It does not have to be precise as all you need is a start with a similar shape.
  5. After the new thermocouple has taken the shape of the old one, attach it to the pilot assembly bracket and fasten it with the nuts till it is just snug. Do not overtighten, though.
  6. The end fitting should then be threaded onto the control valves and tighten by hand. You can then use an open-end wrench to fasten it. Again, be extra careful not to overtighten it.
  7. Turn on the electrical power and gas supply to bring the furnace back online.
  8. Turn on the furnace’s pilot light and ascertain that about a half-inch of the new thermocouple is within the pilot flame.
  9. Replace the access cover that you had initially removed.

Your thermocouple is successfully replaced and up and running again and more efficiently. However, it is important that you test for any possible leaks after completing the replacement process, just to be sure that gas is not leaking.

Replacing an Electrical Flame Sensor

Similar to a thermocouple, replacing a flame sensor can be a hard task and should usually be left to the pros. However, this does not make DIY’ing impossible. By following the following steps and guidelines, you can address this issue safely. Again, follow the instructions outlined by the manufacturer of that specific model you are about to replace.

  1. Turn off the electrical power and gas supply to the furnace. If the furnace was in use, you should give the flame sensor about thirty minutes for it to cool. Proceed to remove the access cover.
  2. Confirm that the old flame sensor is actually removable. Removable sensors will be secured to the furnace’s burner assembly with screws. If the sensor is not removable because it’s integral to the entire furnace system, consider calling professional furnace technicians to fix the replacement for you.
  3. Disconnect the electrical wires leading to the sensor and those leading to the control box as well.
  4. Proceed to remove the sensor by loosening the screws with a wrench or a basic screwdriver.
  5. Fit the new flame sensor in the old sensor’s position, then secure it tightly with screws or nuts.
  6. Reconnect the wire leads to the flame sensor and the control box to return the flame sensor back online.
  7. Finally, replace the access cover that you had initially removed.

If you are stuck or feel like avoiding DIY altogether, the professionals at Whipple Service Champions are the team to call for help replacing your flame sensor.