In furnace troubleshooting part 1, we talked about how to determine which efficient furnace you have as well as determining your pressure switch problem on your furnace. We also discussed the thermostat and the low voltage circuit connected with the thermostat. Hopefully that was helpful.
This segment of furnace trouble shooting, we will talk about your furnace flame sensor low voltage safety circuit.
What is a flame sensor? A flame sensor on your furnace does exactly like it sounds — it senses flame.
This is a safety for the furnace. If no flames are sensed, the circuit control board shuts down the flow of natural or propane gas so that the fuel does not build up which can cause an explosion. The old standing pilot furnaces had a thermocouple which many people probably remember. That thermocouple did the same thing as the flame sensor; it sensed flame and produced a millivolt (around 24 to 26 millivolts) which kept the pilot light of the gas valve opened.
The flame sensor senses flames as the flames from the burners impinge on the flame sensor. If the flame sensor is operating properly, it will produce micro amps of electricity back to the circuit control board telling the gas valve to stay open and keeps the fuel flowing for the burners to stay on. (Each furnace manufacture has different micro amp voltages for their particular unit).
Now, here are a couple of things with furnace flame sensors that we have found out over the years. If your furnace is in a utility room near the washer and dryer and you use powdered laundry soap, it has been proven that the powdered detergent as it is poured out into a measuring cup or directly in to you washing machine floats in the air and has been know to cause low flame sense shutting down the gas valve circuit. The detergent has impinged itself on the flame sensor. You may say to yourself how can that be? Wouldn’t the flames from the burner’s burn up the powder? The answer is no. What it does is create a coating over the flame sensor causing the flame sensor not to send the micro amp voltage back to the circuit control board thus shutting down the burners.
Other items that can cause the flame sensor not to work are dust and dirt, open or cans of paint near the furnace, and any other aerosol’s and chemicals that can vaporize and impinge on the FLAME SENSOR. Move those items well away from you furnace. It is a safety hazard.
If the burners come on and within a few seconds go off, it is probably that your flame sensor is dirty or defective. As a homeowner, you can clean your sensor for temporary relief until you can call Around the Town Heating And cooling out to replace your flame sensor.