Its possible to see a crack long before it blows open and starts dumping into the plenum/vents .
I went through this with our local trane supplier in omaha ne years ago(oconner co)
They would argue that it wasnt cracked under warranty till it was blown out completely so when Id take them a cracked heat exchanger in to get the warranty credit for it Id circle the crack on outside with a big ol sharpie.
I told them I wasnt going to be responsible for somebody dying just because they wanted to be cheap and not stand behind a major design flaw in their 80% furnaces!
