Same way I clean my windshield.
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It is possible that it was not firing at all. She may have heard the draft fan kicking on and thought it was the burner.
It has been fine since.
I believe the flue is getting a ram air effect due to how it is positioned and angled.
Attachment 73750
This thread was a good reminder for me to buy a spare hot surface ignitor to have on hand . . . arrived today.
And then . . . when I went to put it in the furnace room, I found the other spare that I forgot I had. Better to have one too many, I suppose.
Is that considered a case of 2-is-1 and 3-is-2?