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Kilowatt Heating, Air Conditioning and Electrical Blog

Understanding the Flame Sensor in Your Furnace—And When It Needs Repairs


We like to reassure our customers that gas furnaces are built to high standards of safety, and it’s not difficult to keep a gas furnace running without health hazards. As long as you have your furnace regularly maintained and schedule any repairs it may need promptly, you shouldn’t have any concerns about running the heater safely.

One of the key safety features in your gas furnace is the flame sensor. We’ll examine what this part does to maintain safe furnace operation, as well as when to know you need to call for heating repair in Van Nuys, CA to fix or replace it.

The Job of the Flame Sensor

A gas furnace generates heat by igniting the combustion gas from the burners. In the standard sequence, gas flows to the burners, then the electronic ignition system creates a spark or a hot surface that causes the gas to ignite. 

It’s possible, however, for the gas from the burners to fail to ignite. This creates a potential danger of unburned gas flooding into the combustion chamber and a hazardous explosion. Here is where the flame sensor does its job. The sensor detects if the burners have ignited. If flames are present, the sensor creates an electric current sent to the control board. As long as the control board receives this current, it keeps the gas line open. If the sensor doesn’t detect flames, the control board will shut off the gas flow to prevent flooding the chamber with unburned gas. 

The Faulty Flame Sensor

Here’s some good news about a faulty flame sensor: it will still do the safety part of its job, it will just do it too well. If the burners ignite, but the sensor cannot detect them (likely because the sensor is dirty), it won’t send an electrical signal to the control board and the gas flames will shut off. The safety measure is still working, but the faulty sensor will prevent the gas burners from staying lit for long. The flame sensor could also lose its electrical connection to the control board, in which case the same thing will occur: the control board won’t receive the expected electrical current and will shut off the burners.

When to Have Repairs for the Flame Sensor

You can probably guess the major sign that you have a faulty flame sensor: the furnace keeps shutting off early! When the flames in the furnace turn on but soon shut right back off again, you could be dealing with a flame sensor that’s developed carbon build-up or has lost its electrical connection. This isn’t the only possible cause for the furnace shutting down early, so please don’t attempt to do anything about the flame sensor on your own. The best step to take is to shut off the furnace at the main furnace switch and then call for our technicians to inspect the furnace. We can run special amperage tests that will tell us if the flame sensor is at fault. If it is, we can clean the sensor or replace it, depending on the situation. 

Call our certified service technicians 24/7 for exceptional customer service! Kilowatt Heating, Air Conditioning and Electrical serves Los Angeles and the San Fernando Valley.

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