I have a Payne 90% efficiency furnace, model 398AAZ upflow that is approximately 15 yrs old. I've had a professional heating co. inform me that the heat exchanger is cracked and exhaust is being returned out of the burner assembly. I (and several others) have smelled the odor and know the problem exists. Is there anything other than a crack in the heat exchanger that could cause the excess exhaust to be venting through the house? Such as an improperly vented terminal to outside?(which leaves excess exhaust in the chamber?) It has been explained to me as a KNOWN issue but I want to be CERTAIN before I opt for an entirely new furnace.

Incidentally, any opinions on Rheem 90% efficency would be appreciated as that is the unit I'm considering.