Furnace Flame Sensors: What you Need to Know


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If you’re reading this post, chances are that your furnace flame sensor has gone bad. This post will go into depth about how Furnace flame sensors work when to replace them, and their cost. This will help readers know what they are getting themselves into if they choose to purchase this piece of equipment.

What are Furnace Flame Sensors?

When gas is supplied to the furnace, it must pass through an orifice that provides a small amount of restriction in order to create enough back pressure for the pilot light.

This allows you to have your heater on demand when needed without having it constantly running.

When the temperature inside reaches that of the thermostat setting then electricity flows and ignites this pilot flame into an active flame that turns off at full heat by itself.

The Furnace Flame Sensor senses how hot these are getting so if they are too high, meaning something may be wrong with them, they will shut down until everything cools down again before allowing for another ignition cycle.

Furnace Flame Sensors are a safety mechanism for furnaces. If the flame in your furnace goes out, there is a high risk of carbon monoxide poisoning to you and your family from incomplete combustion because gas can leak through cracks in the flue pipe when this happens if it isn’t burning properly.

In order to prevent that from happening, Furnace Flame Sensors monitor whether or not actual flames are present by using infrared light beams that bounce off the surface of tiny metal plates installed on both sides of every burner tube inside your furnace.

When these sensors detect even one beam being blocked within five seconds it means that there is no fire going on in the hot zone where heat gets generated in an unlit burn cycle which will cause the power to shut off.

Furnace Flame Sensor Replacement

If the Furnace Flame Sensor is replaced it should be done by a licensed professional who knows how to safely remove and replace them without damaging the other parts in your furnace because if they don’t know what they’re doing, you could end up with damage that requires even more expensive repairs than just replacing this one part did. So, make sure you buy only quality products from trusted brands.

It’s always better safe than sorry when it comes down to preventing dangerous situations such as carbon monoxide poisoning. Now we can talk about their cost: Since these are fairly new technology sensors, there isn’t really any information out there regarding repair costs or replacement costs so I’ll just give you a general idea.

Flame Sensor vs. Thermocouple

Furnace Flame Sensors are not to be confused with Thermocouples which have a different style of working.

A thermocouple is used for sensing temperature levels whereas Furnace Flame sensors sense whether or not there’s an actual flame being produced inside the furnace by monitoring infrared beams that bounce off metal plates installed on both sides of every burner tube where heat gets generated during combustion.

Now we can move on to replacing Furnace Flame Sensors.

When To Repair or Replace?

The best way to tell if you need any type of furnace repair is either through trial and error because your heater doesn’t turn on but usually, it will stop blowing hot air out of vents before that happens, or by waiting until temperatures drop below freezing and see if this problem still occurs when heating element heats up again. If it does, then you may need to replace your Furnace Flame Sensor.

With that being said, if these are not working properly it can also be caused by dirty or rusted burner tubes so before replacing the part itself first thing, I would do is have a professional clean out my furnace system since it’s possible there could just be some dirt blocking the infrared beams from getting through causing them to shut off at random intervals for no reason whatsoever.

If this doesn’t solve anything after doing everything on your end first then of course go ahead and purchase one because they’re fairly inexpensive components.

Signs of a Bad Flame Sensor

If you are curious about whether or not your Flame Sensor is bad, then there are some signs that will let you know.

If it’s dirty and blocked by dirt or dust on the metal plates inside of burners where heat gets generated, this creates resistance preventing beams from being able to bounce off which means Furnace Flame Sensors won’t work properly if at all so they may shut down unexpectedly even though everything else seems fine with them.

A second sign would be detecting black carbon deposits around air vents caused by an unlit burn cycle when gas was leaking through cracks in furnace flue pipe because no flames were present to use up extra combustibles such as oxygen which can cause damage requiring expensive repairs over time.

Flames turning off randomly is another sign that these sensors are not working properly so it’s highly suggested to have them checked out by a licensed professional before the problem gets worse over time because Furnace Flame Sensors work as an important safety mechanism for any homeowner who uses furnaces in their house since they prevent dangerous situations such as carbon monoxide poisoning which has no warning signs whatsoever. So be proactive, know what to look for, and make sure everyone stays safe!

Furnace Flame Sensors Cost

In terms of cost, Furnace Flame Sensors are fairly inexpensive parts. They can be purchased online or from a local hardware store if you prefer. Usually, these sensors last anywhere up to twelve years depending on your usage.

However, they will eventually wear out over time which is why it’s recommended that homeowners replace them every ten to fifteen years maximum in order to prevent dangerous situations such as carbon monoxide poisoning which has no warning signs whatsoever so again, make sure everyone stays safe!

In addition, the price varies depending on your furnace type and brand. We recommend checking the site of a brand you are interested in to see the price of this specific part.


All in all, Furnace Flame Sensors are a great safety mechanism. It can help prevent serious health problems such as carbon monoxide poisoning.

As long as they’re installed and maintained properly by a professional, you don’t need to worry about these malfunctioning or causing any other damage like dirt in your furnace system blocking infrared beams from getting through.

If you’ve been experiencing issues with this part of your heating system, make sure to talk with us today because we have the best team of professionals at Congressional HVAC waiting for you!



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