How Does A Flame Sensor Work?

Every gas furnace has a flame sensor. If the flame sensor fails, the rest of the furnace will not work – until you repair the sensor.

So, what exactly is it, and how does it work? We answer these questions below and discuss several other things you need to know about the furnace flame sensor.  

What is a Flame Sensor?

The furnace flame sensor is a device found on home furnaces and large industrial boilers to determine the presence of a flame. They are part of the heating’s systems safety circuit and work alongside the control board to prevent gas poisoning and potential fires if the furnace is ON without a fire. 

What Does a Flame Sensor Do?

The primary purpose of the sensor is to detect the presence of a flame in the burner. By so doing, it guarantees that the gas headed to the burner is burning something.

Otherwise, the gas would flow out of the burner(s) into your home, creating the risk of gas poisoning and potential fire. 

So, first and foremost, it’s a safety feature included in the furnace system to protect you and your loved ones.

Additionally, the flame sensor; 

Protects the furnace from flame rollout damage

The flame sensor also protects the furnace from flame rollout. Rollout refers to the tendency of gas furnace flames to roll beyond the combustion chamber. The leading cause of flame rollout is concentrated combustion gases followed by a compromised chimney.

However, gas lingering above or outside the combustion chamber at ignition can also cause rollout. Rollout can damage the combustion chamber and poses a serious fire risk. 

Controls furnace ON/OFF operation

Above all, one may also argue that the flame sensor controls when the furnace comes on. Although the furnace’s ON/OFF control involves many other parts and components, if the flame sensor says NO, the furnace will remain OFF or go OFF if previously running. 

How Does a Flame Sensor Work?

The flame sensor works by sending a small electrical signal to the central controller when it senses a flame. Central controllers are the motherboards present in heating systems. They are in charge of every activity on the heating process, including the gas supply to the burner. 

When the controller receives the electrical signal from the flame sensor in many furnaces, it immediately puts the sensor on a timer. During this “ignition period,” the furnace ignores all readings from the sensor until the sensor verifies the presence of a large flame. 

After the timer has elapsed, the furnace continually reads signals from the sensor to monitor the combustion process. 

Metal Strip vs. Optical Flame Sensors 

 Furnace flame sensors come in two broad categories, i.e., metal strip sensors and optical sensors. 

The metal strip flame sensor is a bimetallic strip that puts out a small voltage in the form of a few millivolts, read as 0.001volts or 1.0mv. When the sensor comes in contact with a source of heat, it generates a small current. The stronger the heat, the larger the current. The sensor then sends back this charge to the control board.

Metal strip sensors are the most common type of flame sensor in domestic applications. They are relatively inexpensive, typically costing under $40. They are also highly accurate at detecting flames as the two metal pieces are electrically opposed meaning any little charge will cause an electrical current. Many heaters usually amplify the charge to make it to the control board. 

Meanwhile, optical flame sensors are generally used in industrial applications. These sensors can “see” a flame. Also, they generate a much larger charge, typically between 2.0V and 5.0V (DC).

An optical flame sensor uses a photocell to detect the presence of a flame. When a photocell is exposed to ultraviolet light, it generates a voltage. The stronger the light, the higher the resulting voltage. 

Although highly accurate, the main downside of optical flame sensors is that they’re susceptible to dirt and debris. As such, the sensor can malfunction at the slightest exposure to dirt.

Additionally, the sensor window must have a complete view of the flame. Otherwise, it may not accurately assess the flame’s intensity, which may cause erroneous readings and misleading feedback. 

Where is the Flame Sensor on a Furnace?

Finding the flame sensor on your furnace shouldn’t be too difficult. But before you even begin to look, you must know how it looks like. 

As we saw earlier, most home applications (and appliances) use a metallic rod flame sensor. The metallic rod flame sensor is a thin metallic rod that sits in front of the pilot flame on the interior of your furnace. The sensor typically has a porcelain base and thin wire sticking from the head into the area where the pilot flame typically goes. 

To locate it, you must first remove the furnace access cover. Depending on the furnace model, you may need to remove a few screws or bolts to release the cover. 

Once you remove the cover, the flame sensor is located just outside the burner assembly. The porcelain end connects to the burner while the wire end dips into the flame area. Remember that the white porcelain base can yellow with time or even turn to a dingy brown. 

Most flame sensors are straight. However, others bend at 45 degrees and others at 90 degrees. The bending is perfectly normal and doesn’t affect the functioning of the sensor unless you find that the wire which is supposed to dip into the flame ends elsewhere. 

How Much is a Flame Sensor for a Furnace?

The cost of a flame sensor varies from place to place and one brand to the next. However, the majority of flame sensors are priced between $6 and $75. 

How Long Do Flame Sensors Last?

There’s no one correct answer. Generally, HVAC professionals recommend flame sensor replacement every 2-3 years. However, this doesn’t mean that you must replace your sensor every three years, even when it’s working well. It’s pointless and an unnecessary cost.

Instead, a better question is, “how can I know that I have a bad furnace flame sensor?” That’s because a malfunctioned or “bad” flame sensor is a risk to you and your loved ones. 

To this end, we recommend replacing the flame sensor only when you determine that it’s no longer functional. Regularly checking the furnace for signs of malfunction would be a good first step to help you catch malfunctions early, though scheduled professional repairs are also necessary. 

When to Replace a Flame Sensor

So, when do you know that you have a bad flame sensor? What are the signs that your flame sensor has malfunctioned and that it’s time to replace it?

  • The furnace lights up then shuts down after a few seconds: The furnace can go off for many reasons, including lack of fuel supply or blocked burners. However, it’s most likely a compromised flame sensor when it lights up and goes out after a few seconds.
  • The porcelain on the flame sensor is cracked: The cracked porcelain is usually a sign of internal damage. It mainly results from charge overflow (overloading) and causes short-circuiting within the flame sensor. Short-circuiting means that the sensor may no longer transfer current to the circuit board. So, the board cannot request fuel.
  • The flame sensor is sooty or corroded: Corrosion and buildup of soot can also be signs of a damaged furnace flame sensor. Corrosion usually results from exposure to external elements and may affect the sensor’s capacity to “feel” a flame. So, it may return a “no flame” answer even if a flame is present. Soot and dirt buildup also impact the sensor’s sensitivity.

How to Test a Flame Sensor

However, before you replace the sensor, it might be beneficial to test it to prove beyond doubt that it’s indeed damaged. You need a multimeter for this process. Once you have the multimeter, follow the steps below;

  1. Ensure the furnace is grounded correctly: With the furnace turned off and using the multimeter, take an ohm reading between the neutral and the burner assembly. It should be very little, i.e., close to zero. 
  2. Connect the multimeter to the flame sensor: Connect the multimeter leads in series after ensuring the flame sensor rod is in the current position within the burner assembly. This means you need to disconnect the wire from the rod, connect one multimeter lead to the rod and connect the other to the wire you removed from the rod. 
  3. Take the resistance reading: When the burner ignites, you should get a reading between 0.5 and 10 microamps, though readings between 2-6 are the most common. 

If you get a reading below 0.5 microamps, you might have a bad flame sensor rod. It might be time to replace. 

Furnace Flame Sensor Cleaning

However, if you get a good reading, ideally within 2-6 microamps, the flame sensor might be dirty. 

To clean the flame sensor rod:

  1. Remove it from the burner assembly and scrub it gently using fine steel wool to remove any dirt buildup.
  2. Test it again to see if it works well now.
  3. If it doesn’t, replace it. 

The flame sensor is a critical part of the burner assembly, without which the furnace cannot work. Therefore, you must keep it clean at all times and replace it promptly to keep the furnace running smoothly.