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How does a flame sensor work

Are you wondering how a furnace flame sensor works? What does it actually measure? Or how does a Lennox flame sensor function? Which type of flame sensor is the best for you? Do you want to know how a flame sensor works? Are you tired of explaining how it works to your neighbor and want an expert to help? Well, you’ve come to the right place. We use flame sensors in furnaces and other heating devices as a safety measure. When there is no flame, these devices will turn off the power source until the sensor reads that there is a flame.

No matter the type of furnace you have, one will install a flame sensor near the burner. This sensor detects the presence of a flame and sends an electronic signal to the furnace’s control. From there, the signal activates the ignition circuit. If, for some reason, there is no fire – like after hours or if an animal has crept into your ducting system – then your furnace won’t operate and heat your home (or business) on its own.

What is a flame sensor?

A flame sensor is a device for detecting the presence of a flame/fire, and it can be either an electric or a photoelectric one. For example, a flame sensor is an important component in a furnace. It is used to detect the presence of the flame and shuts off the furnace when it is not present.

The sensor detects the presence of a flame by sensing infrared radiation emitted from the flame. When it does not detect any infrared radiation, it will shut off the furnace to prevent overheating and possible damage to other components.

The flame detection response may depend on its fitting, using an alarm system, natural gas line, and propane. When fitted to a natural gas line, they can activate an alarm system or stop the gas supply. When installed on propane tanks, they work as a safety measure by sounding an alarm or shutting off the gas supply if it detects fire or flame. Industrial boilers use thermal sensors to automatically authenticate whether or not it is properly working. These sensors’ response time and accuracy are faster and more accurate than a smoke/heat detector. For this reason, we use them to stay on top of the boiler’s performance.

The device consists of a temperature-sensitive material that the flame must heat to operate. When the material senses that it is warm, it will produce an electrical signal normally set at or near 10 millivolts. This signal can then become useful in activating various devices such as an ignition transformer, relay, switch etc.

Understanding the Furnace Flame Sensor

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The furnace flame sensor is a specialized safety component present in most conventional gas-fired furnaces. The furnace flame sensor is a diagnostic device mounted on the furnace blower that provides information about whether the flame is too large for safe use. In addition, they detect and confirm the existence of a heating flame inside the furnace’s burner assembly. When no flame is detected, the furnace flame sensor will turn off the furnace to prevent possible leaks of hazardous gases like carbon monoxide. This will keep you safe and is also preferable in avoiding potential explosions.

You place the flame sensor on top of your furnace to detect any flames present in that particular area. It does this by using infrared radiation and high-frequency sound waves. You must pair this type of sensor with other sensors, such as the photoelectric and magnetic effect sensors used for most residential furnaces.

Sensors can be present in both non-automatic and automatic furnaces. The pressure from the fan determined by the sensor controls the amount of oxygen fed into a furnace’s combustion chamber, increasing or decreasing it as needed to reduce the risk of carbon monoxide build-up.

How does it Work?

A furnace flame sensor works by measuring the temperature of free radicals in the air, resulting from burning fuels such as natural gas or oil. The higher the oxygen content, the lower the emissions produced by a controlled fire. The design of a furnace flame sensor is unique to sense when there is a proper amount of oxygen in a controlled fire by measuring the amount of free radicals produced. When oxygen levels are ideal, the temperature of free radicals decreases, and when it isn’t, then free radical temperatures increase. This change in temperature activates the furnace sensors and switches off the burner when it senses an unsafe environment.

A furnace flame sensor detects the presence of a flame by sending out an electric current. The thin metal rod creates this current when it senses heat present in the furnace. When the gas valve opens, heat detectors are activated to tell if there is a flame.

Once you turn on the furnace, it will keep running as long as there’s a fire present. However, if 10 seconds go by without detecting any flame from the furnace, the system will shut itself off. This means that the heating will cycle 3 times before shutting down the control board and locking out the system. You can try a few things on your own, or perhaps you will need to call an HVAC technician to come reset it. These safety measures help keep you, your friends, and your family safe.

Furnace Ignition Sensor

A furnace ignition sensor is a device that we mount on the burner assembly of a gas-fired furnace. It can also be a part of a burner assembly Its purpose is to ensure that the burner ignites and will turn off when the flame goes out.

There is a connection from the sensor to an earth wire to prevent interference from the burner assembly. Plus, you should install a carbon monoxide detector nearby for added protection against CO poisoning.

The function of a furnace ignition sensor is to sense if there is adequate oxygen present before allowing the ignition of the gas or oil burner. There are two types of furnaces that use this sensor, which include the pilot-ignited and direct-fired furnaces. Pilot-ignited furnaces contain a small burner that lights the furnace after it undergoes ignition by an electric spark. A flame sensor is responsible for the ignition of the electric spark. Direct fired furnaces are lit by electrical coils or directly from gas or oil burners.

Over time, if there is no proper cleaning of the flame sensor, you could experience issues that may impair the functionality of your furnace. For this reason, you must clean it regularly.

Some furnaces have separate sensors they use instead of the ignition sensor. They come in handy when there aren’t any gas valves in the furnace’s gas line.

The Lennox Flame Sensor

A Lennox flame sensor comprises the same components as any other type; however, their design makes them more efficient than regular sensors. As a result, Lennox flame sensors have a longer life span and can withstand high temperatures without showing signs of damage. In addition to these benefits, the Lennox sensor has a thinner profile and can fit into tighter spaces than other types.

A Lennox flame sensor is installed on the furnace blower and signals the blower to increase or decrease its speed when it detects the flame’s temperature. There is mounting of the flame sensor on the blower housing of Lennox furnaces. It sends signals to a secondary control board that adjusts gas and air feed rates to burn safely in relation to surrounding objects.

If it’s too hot or cool, it sends a signal to adjust the airflow so that combustion occurs at a safe temperature.

This is especially helpful with furnaces having multiple burners or large burners because excess heat can negatively affect comfort and efficiency.

Furnace Ignition and Flame Sensors Troubleshooting

Ensure that only qualified professionals service furnaces. If the flame sensor needs replacing or cleaning, a professional should do this for best results.

However, it shouldn’t be too hard to do on your own if you have some basic knowledge of how it works. For example, you can take off the blower housing and then the blower belt since both connect to the flame sensor.

Then, look for any dust or debris that may have lodged in the sensor to see if it needs cleaning. Finally, don’t forget to reattach the blower housing after cleaning and testing the flame sensor.

You can also use a multimeter to check whether the flame sensor works properly. First, turn off the furnace by flipping on its switch. Next, use your meter to measure voltage, which is how much current in the wires coming from either one of them (ignition or flame sensor).

If the voltage reading is above a hundred millivolts, they are working properly. However, if you get a reading of less than 100 millivolts or there is no reading at all, then either the wires or the sensor need to be replaced.

Which type of flame sensor is the best for you?

 If you have an older furnace running inefficiently, it may be time to consider a new furnace or at least one with a better ignition system. A Lennox flame sensor will work well with all types of furnaces and help your older furnace run more efficiently.

The Goodman Flame Sensor

The Goodman flame sensor is useful in Goodman furnaces and boilers. You can locate it in the front seams of the furnace, where there is protection from sparks and debris that can impact the flame sensing element. It follows a design that senses the temperature continuously and rapidly through a tiny wire coil around the oxygen sensor.

Goodman furnaces use a flame sensor to provide feedback regarding how long the burner stays on.

When the blower shuts off and stops sending air through the burner, this sensor senses that the burner is no longer hot enough to keep going and sends a signal to shut it off.

The flame sensor on your Goodman furnace prevents inappropriate fuel combustion. To achieve the maximum efficiency, always clean the flame sensor as recommended.

A professional HVAC technician comes in handy for furnace maintenance and repair. Nevertheless, it is a simple and do-it-yourself process to clean furnace flame sensors.


Flame sensors are devices useful for diagnostic purposes as well as safety. They are typically present in furnaces, ovens, and other devices that use combustion to operate.

The Goodman flame sensor is no exception. The device helps detect any hazardous conditions within the combustion chamber of your furnace and let you know when the burner needs maintenance or replacement.

While it may be tempting to replace the flame sensor instead of calling a professional, it’s important to realize that this can do more harm than good.

That’s because there is a connection of the flame sensor to a part of the furnace that’s useful in controlling the heat output. It can be highly damaging if not properly maintained and serviced by a professional.

How to Install a Flame Sensor on a Goodman Furnace?

That said, let’s learn how you can install it on your Goodman furnace to get the best results.

First and foremost, you need to find out who installed it. If it was a professional, it should have labeling by the manufacturer.

If you can’t find it, you might need to purchase a replacement since this is one of the most common problems that can occur with Goodman furnaces. Ensure the sellers have verified their universal part number and are willing to sell it to you.

Next, open up the furnace by locating the access panel and removing the screws that hold it in place.

Don’t damage any existing wires or tubing as you remove it and set it aside until it is successful.

Inside, you will find the blower assembly with a piece of cardboard attached to it. Remove this piece of cardboard and look for a connector with three slots in it.

The arrow in the connector should point towards the furnace’s front. Next, remove all the wires from this connector and discard them.

Use your multimeter to test for continuity to confirm that you have removed the wires correctly.

Now, take a glance at your old flame sensor and compare it with your new one to make sure that you have purchased the right model for your unit.

What Are the Advantages of Goodman Flame Sensors?

The Goodman flame sensor is an excellent choice for people looking for an infrared gas sensor that:

Feature Easy installation: You can simply screw the sensors can simply be screwed into place, and then you can be assured that they will function continually.

Fast response time: You will not have to wait for long when the furnace sensors detect issues before responding.

Accurate: Goodman flame sensors work well and can provide reliable measurements for heat control by the furnace or boiler.

What Are the Disadvantages?

The only disadvantage to using this type of sensor is that it can be pricey, which means that you need to be sure you are choosing the right furnace model, or else it may not be as useful as you imagined. Goodman flame sensors are often combined with other sensors because they work well with their performance. For example, you can opt for a flame sensor and an electric sensor for very low fuel pressures. This combination will work great if you want a very precise and accurate readout of the oxygen levels in the interior of your furnace.

Bypassing a flame sensor

Is it possible bypassing a flame sensor? While disappointing, the answer is no: simply bypassing your flame sensor isn’t an option.

It’s not possible to bypass a flame sensor with any simple steps. A number of furnace technicians may claim to understand how to do such for selected furnaces. The newer furnaces don’t allow this anymore. So far, we have no definite way of doing it.

If you’re unsure why something isn’t working, there’s no need to bypass safety controls. However, if these sensors aren’t helping, utilize instrumentation and meters to test them properly.

If you think the flame sensor has failed and there’s a quick fix for it, you may be wrong. It’s usually not a good idea to jump from sensor to sensor because it can cause problems with the other sensors in the room. Unlike the safety features on a computer, the flame sensor plays an essential safety feature. Trying to override it would not be a good idea, get a qualified professional to handle this. Checking with your manufacturer is always the most reliable option.

Telltale Signs that your Flame Sensor has Failed

If your home or business’s flame sensor is not working properly, it could be a sign that something is amiss with your furnace. There are several ways to tell that the flame sensor has stopped functioning well:

  • The furnace stays on for a few seconds before shutting off quickly, suggesting it’s not sensing the correct heat. Or if it sounds like it tries to cycle on without igniting at all.
  • If you see that the flame sensor’s porcelain is broken or cracked, it needs replacement. In addition, you might need to unbolt and detach it for inspection.
  • The flame sensor has a heavy coating of soot or corrosion is severely hampering the sensor’s functionality.
  • Your furnace may not turn on despite the presence of power through the equipment.
  • Suppose your furnace will not shut off even after the room has completely cooled down. Then, you should turn off the flame before you go to bed and turn it on again in the morning.
  • The heating system will automatically turn on but then blast hot air into the room, even though there is no heat release.
  • Suppose you hear noises coming from your furnace even though the valve is off and the room is cool. Noise may be coming from your furnace’s burner, flame sensor, or a relay inside the furnace that converts heat from the burner to electricity.

What should you do?

One of the possible signs that you need to clean or replace your flame sensor is a furnace displaying an error code. You can usually tell whether the error code occurs because of a clogged sensor by looking at the symbol on the furnace, which will either be blinking or solid. An error code can be a number, a combination of letters and numbers, or an on-screen symbol. Sometimes when your flame sensor is malfunctioning, it will display the word “LOCKOUT.” If you see that, then it means that there was no fire in the oven, and we need to reset your oven while we order a replacement.

If you notice any of these potential problem areas, we highly recommend you contact your gas provider so they can send a specialist to check the unit.

Without the flame sensor, can your furnace work?

It seems that you might still have some questions about what we are sharing here. Check this out.

Flame sensors are common with furnaces, you can usually install them in the venting pipe. If there’s a flame, it means that the furnace is operating and there is burning of the gas. However, if there isn’t a flame present, there is a gas leak with no flame sensor.

Your furnace draft motor automatically activates when the thermostat requires heat, and your igniter heats as soon as it does. Gas is released as soon as the valve opens, and you have a flame. Flame sensors detect flames. As a result, you will have a properly functioning furnace, and your house will be warm as expected. If your flame sensor fails, the furnace shuts off automatically, and the gas will stop flowing. A house minus a flame sensor and a flame would not be ready to handle an explosion.

In other words, a furnace will not function minus a flame sensor. A flame sensor’s primary function is keeping you and the entire family safe and allow the furnace to work properly.

How a flame sensor works with other types of ignitions

When there are multiple ignite sources in a home or business, the electronic signal sent to each igniter may not be the same. In this case, the furnace will not run, which could leave you and your family without heat. To solve this problem, each ignitor must be programmed to recognize the flame sensor signals from each other. Many furnaces come with a burner and an ignitor pre-programmed to work together. On choosing your specific furnace model, you get the listing on your product’s datasheet or the associated manual.

Thermocouples are another common type of thermostat for a furnace. These sensors use the principle of thermoelectricity to ignite the gas. When you press the thermostat, there is a voltage release in a circuit similar to what you would find in a galvanic cell. The current is sent from one thermocouple terminal to another and completes the circuit. This causes an electrical signal to flow through your furnace, which activates every part of it that supplies heat or air into your home (or business).

How a flame sensor works with other furnaces

Heating systems that contain a flame sensor also have an ignition circuit. Once the ignition circuit is activated, it will produce a spark at the right moment to ignite the burner. The ability of your entire system depends on two things:

  • the amount of voltage your furnace’s control move and
  • whether or not the voltage is sent to all of the parts of your furnace that it needs to operate


Having a flame sensor will ensure that your furnace and air conditioning system works as intended. Thus, nothing gets in the way of producing heat or cold air when you need them the most. If a flame sensor is not in place, your furnace might not produce the proper amount of heat or cold air. This may cause you to use more energy than you need. A flame sensor ensures your heating system does not turn on and off, even when power is flowing to it. If a relay fails inside your furnace and turns off the power in transmission, the heating system will continue to run until the room cools down, which could result in a fire or explosion.

Replacing a flame sensor is not too complicated, but it may take you some time and effort to do. Such depends on your knowledge of your home’s heating system. Always turn off the power to your heating system before doing anything else. Make sure that it will not turn on by itself before you begin working on it. If you need help replacing a flame sensor, contact a professional technician like Rayming PCB & Assembly. We can help you determine the proper replacement part, no matter what brand your furnace is. So give us a call today and get started restoring your heating unit to its full functionality!

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