Furnace Flame Sensors 101

Furnace Flame Sensors: What You Need to Know

If your furnace is powered by natural gas or propane, you should be aware of its furnace flame sensor, an important safety feature. Understanding its function and how to maintain this sensor can prevent sudden furnace shutdowns at very inopportune times.

The furnace flame sensor makes contact with the flame in your furnace’s heating element. As long as it senses flame, gas flows to the elements. If the sensor does not detect a flame, it shuts the gas valve.

Without this feature flammable gas could continue to flow into your home, creating the potential for poisoning, an explosion, or fire. This guide will help you understand its importance, where it is located, and how to clean or replace your furnace flame sensor.

What to look for.

If you remove the furnace access cover, you will see the burner unit with multiple elements. Adjacent to one of the elements you will see wires leading to a ceramic insulator. This is the furnace flame sensor.

The sensor includes a small stainless-steel rod that extends into the flame while the burner element is engaged. It may be straight or may bend 45 to 90-degrees to touch the flame. The insulator keeps the heat from radiating to the sensor and wiring, since the flame may burn as hot as 3,000⁰ F. The sensor is located inside of the ceramic surround and if it registers heat, the furnace should function normally. If the sensor does not detect heat from the flame, it closes the gas valve.

• Natural gas is hard to detect; without the flame sensor, the explosive gas could accumulate within your home before being discovered.
• While natural gas is non-toxic, it can displace oxygen in the air and lead to suffocation.

“Isn’t that a thermocouple?

Some of us remember a safety feature from the past and confuse a flame sensor with a thermocouple. What is the difference?

• Older model gas furnaces featured a small flame—a pilot light–that burned continuously and served as the ignition source for the burner elements. A thermocouple was positioned directly above the pilot light, detecting heat.

  Should the pilot light be extinguished, the thermocouple shut the gas valve to prevent the accumulation of gas within the home.

• Furnaces with electronic ignition do not require a pilot light and are more energy-efficient. When heat is called for, the gas is ignited electronically. The lack of heat from a flame signals the sensor to shut the gas flow to the burner elements.
• Both thermocouples and flame sensors are safety features designed to accomplish the same task. The age of your furnace will determine which feature is used.

How can I tell if the flame sensor is bad?

Every mechanical device will need some maintenance and every sensor will eventually need to be replaced. If your furnace is not functioning properly, here are some indications that the furnace flame sensor might be the problem.

• If your burner unit ignites but shuts off after a few seconds, it might be the flame sensor.
• If after you remove the access cover and locate the flame sensor and see
  • A cracked or broken ceramic insulator
  • The tip of the flame sensor appears misshapen or burnt on the end
  • The sensor appears to be covered by black soot or a dark covering

If the furnace flame sensor appears to be the problem, we will give you step-by-step instructions for removing, repairing, or replacing the flame sensor. Refer to the furnace user manual/parts guide to research the appropriate sensor for your brand and make of furnace.

Expect to pay less than $20 for the part. The service call to have it installed by a professional HVAC tech will range in price from $75 to $250.

How long do they last?

As you can imagine, a metal rod inserted into the flame of a furnace will have a limited lifespan. Expect the flame sensor to last approximately five years, a little longer with some TLC. A well-maintained HVAC system has an average lifespan of 15 to 30 years, so your flame sensor will need to be replaced several times during the life of your system.

What about this TLC? Annual furnace maintenance is proven to extend the life of HVAC systems. A part of this service would be an inspection of the flame sensor and cleaning or replacing it as needed. Removing the soot build-up over a season of use will lengthen the life of the sensor.

A do-it-yourself guide for cleaning and replacement

If you have a DIY spirit and a few tools, you can clean and/or replace the furnace flame sensor on your HVAC system. Make sure to purchase the proper component for your system and have the sensor in hand before you take any of the steps below.

1. Turn off the electricity to your unit by flipping the furnace breaker at your electrical service box. This prevents electric shock should you touch something you should not.
2. Turn off the gas supply to prevent the burner element from igniting unexpectedly. Notice the gas valve on the supply line.
3. If the unit has been running recently, wait until the sensor has cooled—at least 30 minutes. Remember, the flame burns at close to 3,000 degrees Fahrenheit.
4. Refer to the operation manual to locate the flame sensor on your unit.
5. Remove the furnace access cover by removing the screws holding it in place. Lift the cover off and lay it aside.

If you have followed these steps, you have created a safe workspace and can proceed.

1. The flame sensor is held in place by a screw or bolt. Unscrew it and grasp the ceramic insulator. Gently remove the sensor, remembering the stainless-steel rod may have an angle.
2. Disconnect the wire from the sensor. This is the reason to follow step #1.
3. Observe the ceramic insulator, looking for cracks. Observe the stainless-steel rod, looking for soot build-up, loss of shape, or evidence of melting.
4. If the sensor maintains its integrity but is covered in soot, you can clean the soot with an emery cloth.
   1. Make sure not to touch the rod. The oils from your hands will burn in the flame and deposit soot on the rod.
   2. Aggressive cleaning will shorten the life of the sensor; gently remove the soot—no more.
5. If the sensor has lost its integrity or appears to be burnt, it will need to be replaced. At this point, you are glad you purchased the appropriate sensor before you began. If the sensor needed to be cleaned but not replaced, you will have the sensor ready when needed.
6. Replace the sensor, remembering the angle. It should have guides to ensure the stainless steel rod is in the proper position to encounter the flame. Replace the screw that holds the sensor in place.
7. Reconnect the wire to the sensor.
8. Reattach the furnace access cover.
9. Flip the breaker and restore electricity to the furnace. Turn the gas supply valve to an open position.

Test your work: turn the furnace on to see if repairing or replacing the flame sensor solved the problem. The furnace should cycle on and off properly.

If you do not have a confident DIY spirit and prefer to call a qualified service technician, give Dr. Cool AC a call.

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