What Should I Do If I Have No Heat?


Dirty Flame Sensor Problems

It is typical this time of year to notice that your furnace is turning on and off three or four times for very brief runs. Then after the three attempts, the furnace shuts down for about an hour or two. After a this wait period, the furnace tries to start-up again. So why is my furnace not staying on? It is possible your flame sensor is the problem. Do I have a bad flame sensor? Maybe you have a dirty flame sensor. This is very common for furnaces that have not had a tune up or regular maintenance in the last 2 to 5 years.woman wrapped in blanket picture

So what should I do about a dirty flame sensor?

Have a service technician clean and check your flame sensor.  It may need replacing or just a cleaning depending on the problem. Typically either one of these solutions will be a relatively low cost repair. Cleaning and checking the flame sensor should be a regular part of an annual maintenance tune up. Call Alexander Heating and Air for a tune up or get an annual service contract and feel confident that your furnace is in great shape.

Warning: most furnace repairs involve high voltage and flammable gas. It is highly recommended you don’t do this yourself. Have a skilled HVAC technician check and clean your flame sensor.

What is a flame Sensor and why are they necessary in my furnace?

The flame sensor is one of many safety features in a gas furnace. There are some that are checked before the gas is allowed to burn and some after the gas is burning. The ones that are checked after the gas is burning are there to protect you and your home from dangerous conditions while the gas is burning. The flame sensor is one of those safety features. The flame sensor is made of metals that will create an electrical current if in the presence of a flame. It is designed to shut the system down when not enough burning fuel is detected at the last burner in the group of burners in the furnace. A steady flame at the last burner would indicate a safe burning environment and that all the burners are receiving approximately the same amount of fuel and air. If no flame is present, then any one of the burners in the group could be burning fuel in an unsafe way. So the system checks for the flame and shuts down the fuel valve when no flame is detected. Any system can have a false reading so the control board usually attempts to light the burners two more times. If all three times get the same message that the flame sensor is not detecting a flame, the entire furnace is shut down by the control board either for a few hours or completely until reset by a service technician. This is called a lockout condition.

What causes flame sensors to malfunction?

There are two possible problems with flame sensors. A dirty flame sensor is the most likely problem. At this point is it important to note that burning fuels do produce trace amounts of water when burning. And when water contacts most metals, the metals tend to rust. The same is true with flame sensors. The metal end of the flame sensor can rust over time. The rust builds up and prevents the flame sensor from producing the current that tells the control board that it detects the flame.  The other possible problem is that the flame sensor has gone bad. This can be a cracked insulator or a short in the electrical components. Replacement is recommended for a bad flame sensor.


Alexander Heating Air Conditioning
(919) 886-4822



About Author

Frank Alexander is an experienced engineering professional who holds a Master of Engineering degree from North Carolina State University and an MBA from the University of South Carolina.

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