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SouthWest HVAC Blog

What Your Furnace’s Flame Sensor Does for You


One of the most popular types of heating systems is the gas furnace. These systems are familiar to most people, reliable, and affordable, and the natural gas they combust is a less-costly fuel than electricity in most places. They have many components, most of which are used to produce heat. But some of the components are there for another reason: your safety.

Your gas furnace’s flame sensor is one of the components that is part of the safety system. What is a flame sensor? Why is it necessary? What happens if it doesn’t work properly? We’ll tell you all about it.

How Combustion Works in Your Furnace

As your home’s temperature starts to dip below the lower threshold for your comfort, your thermostat sends a signal for the furnace to come on. It receives the signal and allows a tiny bit of gas to flow, just enough for the electronic ignition system to light it with a spark. Now, in order to use that flame to light the burners, more gas needs to flow.

Alternately, in an older furnace, there’s already a tiny trickle of gas flowing all the time. This keeps the pilot light going. There’s no spark needed for ignition because there’s a standing flame always alight.

The Job of the Flame Sensor

Imagine what would happen if the pilot light was out, or the electronic ignition failed to create a spark. With no flame to burn the natural gas, if more of it started flowing out to supply the burners, you’d have a dangerous gas leak! So it’s critical that the increased flow of gas only happens when there’s a flame to combust it.

The flame sensor has you covered. If there’s no flame, it will shut down the system so no gas will leak. How does it do that? Well, it’s really pretty amazing, especially if you know that it’s not a fancy security camera or anything—it’s simply a metal rod. What it does is complete a circuit from the control board, using the flame as part of that circuit! No flame means an incomplete circuit and a shut-down furnace.

If the Flame Sensor Fails

Ideally, the flame sensor would simply shut down the system if the pilot was out or the ignition system didn’t create a flame. But there are times when the flame sensor turns off the furnace even though a flame is present. Why? 

Well, now that you know how the flame sensor works, you can see that it must be entirely conductive in order to complete the circuit. If it’s coated with a layer of carbon residue from the combustion, that can insulate the rod and prevent the circuit from being completed. 

It might seem like a big problem—a shut-down furnace—but it’s one that can be resolved with a simple professional cleaning. Don’t try to do this yourself! For your safety, and to ensure that it’s done properly and without voiding your warranty, have a qualified heating technician do it for you.

Whether you need maintenance, repair, or furnace installation in Long Beach, CA, don’t hesitate to reach out.

Contact us today to schedule your HVAC repair as soon as possible.

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