As a manager for the nation's largest home inspection company, I can say that I have seen my share of differing opinions regarding heat exchangers. Sometimes, the technician that discovers the "crack" is trying to sell you a new unit, knowing you may already have reservations about your current system. I have successfully questioned many inspection reports that called for furnace replacement due to cracked exchangers and upon a more thorogh second inspection proved them wrong. Before opting for a new unit get another opinion. Ask what method of inspection they use to determine the crack exists, if it is not obvious. Typically, a cracked exchanger will result in elevated levels of carbon monoxide in the circulated air. Make sure a CO test has been performed. Another test is with the use of smoke, which when released in the burner chamber, will leak into the circulated air. The most widely used and least accurate method of detection is with a mirror and flashlight to look up into the chamber. What 'appears" to be a crack, many times is not at all. Also, the use of "detection sprays" which change the color of the flame in the absence of a crack are not accurate either. I have blown holes in many a failed inspection by insisting on a more thorough evaluation that includes elevated CO levels. Be wary of any fast talkin' technician that does a cursery test and trys to sell you a new unit. They get big bonuses for that and therefore push their "seasonal maintenance" plans to help sell more product. Good Luck.