WHAT IS THE HEAT EXCHANGER ?

The heat exchanger on a furnace is the metal that separates the fire from the air stream. The problem with a crack in the heat exchanger is that it can let combustion products into the air stream and could cause health problems due to leaking flue products into the air stream. However a cracked heat exchanger is the point at which a furnace is considered to be needing replacement or a new heat exchanger.

If you have a crack on a standard standing pilot furnace with out a combustion blower then you have the possibility of combustion products leaking into the living space but the problem with that is the combustion area is normally under a slight negative pressure and the flames would not go up the tubes but instead roll out. To top it off when the fan or blower is running it puts the outside area of the combustion chamber (heat exchanger) under positive pressure so there is no way the flue products could ever come out. The main reason that flue products would get into the living space is if the chimney is obstructed like a bird building a nest in the chimney and with this you will have other problems too like condensation in the flue pipe. If it happens to be a newer furnace with a combustion blower either an 80% efficient or a condensing furnace 92%+ efficient then there is a combustion blower (or draft inducer) running all the time that there is fire so there is no way that the flue products could get into the main air stream. The biggest problem with having a crack on one of these furnaces is that the flames will roll out because the draft air is leaking through the crack.