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What is the brand and model of your unit?
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The Carrier 58SX series only had a 10 year het exchanger warranty.The units actually had a factory recall because of the heat exchangers.This of course has long passed as far as receiving any benefits. See link:
Even the ones with no defects were good for about 10 to 12 years. So at 25 years the best and probably only solution is going to be to replace the furnace if the heat exchanger is defect and it probably is.
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