A cracked heat exchanger isn’t ideal, but it might not be as dangerous as we immediately assume. If your exchanger is cracked, you’re going to need to replace it. The issue with the cracks is primarily that it might allow carbon monoxide into your home, and if that’s happening you definitely need to replace it as soon as you can. But evidence exists that some cracks don’t necessarily introduce hazardous gases into your home.
Have your furnace routinely inspected, install a reasonably-priced carbon monoxide detector, and look for obvious signs, like corrosion and cracks, strange smells, soot or water build up on the floor. If you see signs of a crack stop using the furnace immediately and call an HVAC professional. They can inspect the furnace and let you know exactly what is going on.