And to be crystal clear, and repetitive, I wouldn't advise continued use of a furnace with a cracked heat exchanger either. That's why I said:
(or notice soot on the walls, or an exhaust smell!)
Part of my point was, you can detect a cracked heat exchanger by the way the flame changes when the blower turns on (there might be a delay, if the crack only opens with thermal expansion). So if you routinely monitor your flame, and know how it looks normally, you will see the change before it gets very far.
Mine wasn't just cut/paste from the Internet, it was one (of many)
reasoned articles, that outlined the physics of the pressure differences between the flame side and the room-air side. I don't argue with physics, it always wins, and it hurts
I'd be very curious on how a cracked heat exchanger resulted in the problem you describe. Are you just relying on the service guy's comment?
The only way I could picture this is with an improperly installed furnace - say the blower was mounted on the wrong side of the heat exchanger, causing low pressure on the heat exchanger side, rather than blowing into, and creating a higher pressure on the heat exchanger side. In that case, the situation is dangerous, and should be remedied rather than just replacing the heat exchanger.
A second way (which would not involve a cracked heat exchanger), would be if you had insufficient return air at the furnace, and a nearby return vent. You might be sucking fumes from the furnace into the return vent and into the room supply. But again, that has nothing to do with a cracked heat exchanger, and the root cause needs to be fixed.
-ERD50