DEAR BARRY: You are part of a large group of folks who misunderstand the effects of a cracked heat exchanger in a forced-air furnace. The supposed "threat" of carbon monoxide is perpetuated by members of the heating industry who have managed to rope in many utility companies and home inspectors in promoting this fear.
When you consider that the heat exchanger is on the outlet side of the blower, it is more likely that circulating air would enter the exhaust stream than exhaust would enter the air stream and the home. This is the conclusion of a new study, but that information has apparently not come to your attention. --Dave