Furnace Maintenance: How to Test a Flame Sensor


Furnace Maintenance: How to Test a Flame Sensor

HVAC experts can often feel like they know the ins and outs of a furnace, but simply knowing the systems won’t keep them running. Standard furnaces require occasional maintenance to ensure that they can continue to operate correctly. Few maintenance steps are more important than testing a furnace’s flame sensor. In this article, we will explore flame sensing and how you can test a flame sensor to keep a furnace operating safely.

What is Flame Sensing?

Gas furnaces are heavily dependent upon flame in order to operate effectively. Without that flame, you will simply have gas dumping into inappropriate parts of the system—a development that can come with fairly significant risk.

Flame sensing is the act of taking steps to ensure that the flame is present and active within the system. This is a core component when ensuring that a furnace is operating effectively, which is why it is essential for every HVAC technician to know how to test flame sensing features. Being able to sense a flame will tell the system whether or not the gas should be active.

Using Flame Sensing Rods

Flame sensing rods are commonly referred to as flame sensors, flame rods, and flame rectifier rods depending on who you work with. No matter what you call these rods, the goal is always the same. These rods play a vital role in keeping a furnace running correctly and reacting to changing circumstances accordingly.

The rod itself is a piece of metal. This metal is fitted with a ceramic insulator, which plays the vital role of keeping the sensor from grounding out. With this device, you can ensure that a flame is present and that your furnace remains safe and operational.

What Do Flame Sensing Rods Do?

A flame sensing rod will create a connection between the flame in the furnace and a furnace board or ignition board. This board will monitor the flame sensing rod while it is actively in the flame—and it makes decisions based on these readings.

When there isn’t a flame, there will not be a current that is relayed to the board. Ultimately, this tells the system that a flame is not present, and it will tell the system what steps to take. If a flame is missing, the system will cut off the gas for a period of time before trying again.

When there is a flame present, the system receives the indication that the system is working correctly. The furnace itself will monitor the flame sensing rod and the flow that is present to ensure that the gas valve should be open.

How to Use Flame Sensing Rods

Using flame sensing rods isn’t something that you really need to manage outside of setup and testing. When using a flame sensing rod, your only goal will be to ensure that the appropriate wires are connected. For testing, you will take active steps to ensure that the flame sensing rod is operational and can be relied upon by the system.

To effectively use a flame sensing rod, you will want to make sure that it is installed correctly. The flame sensing rod should be positioned in such a way that the appropriate piece is fully inside of the flame. Then, the rod will need to be connected to the board so that it can give its readings.

How to Test Flame Sensors

Though the flame sensing rod is a valuable feature in your furnace, it does not always work. Conducting a flame sensor test to ensure that it is working on a regular basis can be a very important step for the system.

When you begin to test your flame sensor, you will want to start by ensuring that the furnace is grounded to avoid any additional risks. The best way to do this is to shut off the furnace during the test until your ohm reading is nice and low.

After your readings are low, you will want to ensure that the polarity of the system is in good order. This means ensuring that hot is connected to hot—and that the same is true for neutral. Then, you will need to double-check that the rod is in the correct position. Remember, the rod must be covered by the flames in order to get an accurate reading of your flame sensor microamps.

When this is done, you will disconnect the wires from your flame sensing rod before connecting your meter lead to the rod itself, as well as the wire that you disconnected.

Activate the burner and check the reading using your meter, which should read in microamps. Refer to the system’s preferred readings range and ensure that the readings you are registering are within the preferred range.

When Flame Sensing Rods Fail

Flame sensors can fail for a variety of reasons. Oftentimes, their failure is linked to a physical issue with the device or surrounding area. Failure might occur because the flame rod is broken. However, they can also run into trouble when the insulator is damaged. In some cases, there can be a more simplistic cause, like a dirty sensor or a problem with the initial installation of the device.

What Does it Mean to “Prove” Flame?

Proving flame is a term that can seem fairly mild until you realize the implications of what can happen if you fail to do so. When you prove flame, you are taking the steps to demonstrate that the burner flame is working correctly and operating within the appropriate ranges. This sequence is essential. Without that proof, gas may be dumped into the heat exchanger. If this happens, it is completely possible for the system to experience significant problems—or even explode.

A Quick Recap

Flame sensing is a very important process in order for furnaces to operate correctly. Taking the time to routinely test flame sensing rods to ensure that they are giving off accurate readings should be done on a regular and as-needed basis. Failing to do so can have dangerous outcomes.

Fortunately, a quick test and reading can help you to ensure that every furnace you check is fully operational. Remember to always deactivate the system prior to testing and be sure to use reliable meters that you can trust!

To properly gather proof of flame, you must be able to test and read flame sensors with complete accuracy, which means that you need the right tools for the job. Buy your [preferred materials] at Shearer Supply today, so you can trust the work that you do tomorrow!