My furnace will light one burner the first time for a few seconds then go out. The second time it will light maybe 3 burners for a few seconds, then go out then the blower will come on...I replaced the flame sensor but no help.
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2What make and model is this furnace?– ThreePhaseEelCommented Mar 2, 2017 at 0:14
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Can you please include more details? For example, make/model of furnace and type of thermostat? It might also help to know how old the furnace is, what size gas line is going to it (if it's on natural gas), whether the combustion air intake is close to an obstruction, how long you've had the issue...– HariCommented Mar 2, 2017 at 0:15
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You may have dust on the burners/venturi which keeps the gas/flame from spreading from one burner to the next. Is the single burner that lights the same one where the flame sensor is mounted?– Platinum GooseCommented Apr 6, 2017 at 19:58
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3 Answers
I had the same thing happen with my furnace and I had to have the blower replaced. A very expensive lesson to learn about changing filters often.
The tech told me that if it tried to start and did not pick up on any air movement it would automatically shut off the furnace.
The lack of full air flow first burned out the blower and then I lack of the blower operating kept the furnace from firing.
As others have indicated, specify your make/model of your furnace. However, based on your short description, I'd take a look at your flame sensor. This is the same exact problem I experienced when my sensor failed (it gets covered in carbon over time). You can test it, by turning your furnace off completely (power and all), disconnect your sensor, "scrape" it with a sharp knife, but don't gouge it at all (your just scraping the black carbon off of the sensor), plug it back in, and let your furnace fire up again. Does it happen again? If not, you need a new sensor (it will likely fail in 1-2 weeks again). disclaimer: If your not at all comfortable with any of this, don't do it, and call an hvac tech.
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I would add to make sure the flame sensor is touching the flame sometimes air flow moves the flame away from the sensor and this can shut down the flow, but usually the pilot will go out when this happens– Ed BealCommented Nov 14, 2017 at 21:54
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Yup, steel wool would be better (not sure why I never thought of that)... Commented Nov 15, 2017 at 1:13
will light one burner the first time for a few seconds then go out. The second time it will light maybe 3 burners for a few seconds
A multi-burner furnace has an ignition source near one end of the burner array and a flame sensor at the opposite end. The flame is supposed to begin at the ignition source, hop from one burner to the next across the array, and ultimately ignite the burner where the flame sensor is located. This has to happen within just a second or two. If it only goes part way across then the sensor correctly detects no flame and causes the gas to shut down.
There is a thin "wing" that connects a small ribbon of gas/flame between the central part of one burner and the next. (This part has a formal name but I don't remember what it is!) Dust can accumulate in this "wing" area, preventing proper flow of gas, which impairs the ability for the flame to propagate from the first burner to the last burner. (photo: airaceheatingandcooling.com)
Often a bit of vibration while the gas is flowing will cause the dust to dislodge. In other words, whack the burner assembly with a wrench or hammer. (Remember the memes about the repairman going into the basement and banging on the pipes?) If it doesn't clear readily, one can remove this formed sheet metal portion and wash it with water. Take care to shake or dry all the water droplets out of that thin wing area; droplets left in there will cause even more blockage than the dust did!
If you do the whack-while-burning method you'll notice that the normally-blue flames flash a bright orange. The change of color is caused by the dust burning. With each successive strike you'll notice less orange in the flame because there's less dust being knocked loose each time.
