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.
Furnace starts to light then goes out
furnace
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I just yesterday dealt with one like that. It had electric ignition (glowing ignitor) and a single yelloow "flame sense" wire. This is only the second one of these I've found in a decade, but the problem and the solution is the same. IF your unit uses ion flame detection, the burner ground has become oxidized. It may look like metal against metal, but a fine coat of invisible oxidation had formed between the metal parts. It needs a good "ground reference" before it can detect the ions.
To fix this, I removed the burner. While I had it out, I scraped the flame rod tip some but it wasn't really dirty. When I put it back in, I intentionally made one of the self-tapping sheet metal screws dig a new groove instead of putting it back in the first grooves. I also got it good and tight.
Just as in the first case, the mere act of disturbing the mechanical assembly caused a better ground to be formed. It must be working, because I didn't have to get up last night and go fix it.
The high limit was indeed being reached. This was confirmed by watching the unit trip the high limit after the burners were turned on.
I had thought that the unit was getting enough air since my filter was new. It turned out that the secondary heat exchanger was packed full of matted dog hair and dust. I'll explain how I cleaned it.
- Remove service panels, disconnect wires (orange stars), disconnect spark plug (pink star). Then remove the two screws marked with the blue stars to removed electrical box, gently set aside.
- Disconnect the 3 wires attacked to the blower, be sure to note how they were connected, then remove the two screws that keep the blower from sliding out on the metal track (visible in the upper left). Then slide out blower.
- Things should then look like this. The secondary heat exchanger is in the back and above the visible foil.
- This is what the heat exchanger looked like, and what it looked like after a bit of cleaning.
I then vacuumed the rest of the heat exchanger as well as everything else to try and reduce future buildup. Everything is then put back in reverse order.
Thank you everyone for your help!
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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.