I'm sure they love the idea of selling me a new furnace even if it isn't
really necessary at this time.

Wondering if other metal inside the furnace were to crack and it weren't the
heat exchanger cracking per se....would that require furnace replacement?

J
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I used to do furnace checks and was told to sell new furnace installs at
any cost short of being fraudulent. The heat exchanger in you furnace is
welded steel and is not known to crack or develop leaks at the welded
seams. However at 20 years in service I would consider an upgrade if it is
your budget to afford it. If it is affordable you would benefit from an
increased efficiency in a high SEER rated furnace.