Actually, that's not the case. :
Myth-1 – Heat Exchanger Cracks | CarbonMonoxideMyths.com
Plus, modern furnaces (even my 80-something % 17 year old unit) have draft inducer fans, actively sucking the fumes up and out the chimney.
That said, a cracked heat exchanger is still a safety issue. If you watch your furnace start, and see a difference in the flame when the blower starts up, that's an indication that there is a crack in the heat exchanger (the blower is pushing air
into the heat exchanger, not the other way around). As the flame is pushed out, it should trigger the flame roll-out temperature sensor and shut down the furnace, but best to detect and fix it before you rely on that safety mechanism.
At the start of each season, and a few times during the season, I'll make a point to watch the flame as it starts, and then when the blower kicks in. I've never seen any marked change that would indicate a compromised heat exchanger.
-ERD50