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Hmmm, carbon buildup on the flame sensor doesn't sound good at all. Typically you'll get a bit of whitish residue that builds up there, not black stuff. I'm not sure what that means but I can tell you that at 20 years that furnace is probably about shot, or at the very least due for a thorough going-over by a pro. One thing you should know: A modern unit is not only much more efficient, not just with heat but with AC too, but with a variable speed fan it is also more quiet and can circulate air 24/7 if you want it to.
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