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.

   But what constitutes a cracked exchanger.  If you have someone look at your furnace to determine if there is a crack in the heat exchanger the best way to do it is visually by shinning a light in the exchanger and looking for obvious cracks. But what constitutes a crack. Many furnaces will have hair line cracks in the metal and sometimes welds will pop and leave a tiny hole. What I would consider a major crack is is the metal has buckled or separated. Sometimes the metal can be of poor quality or condensate from the air conditioning coil can leak and ruin the metal. But when this happens it will usually cause the flames to roll out or dance in the burners. I have seen cases where the heat exchanger is actually rotted out from rust which would the flames will roll out and shut off the roll out switches long before the combustion products can do much damage.

    When having someone look at your furnace make sure that they do a good inspection.

Scott Meenen N3SJH.

1. Where is the crack?
2. How big is the crack?
3. How much air is leaking in out out of the crack?
4. Is the crack actually in the combustion area or in the sheet metal of the frame?
5. How will this crack affect the operation of the furnace?
6. How can this crack cause combustion products to leak into the living space?
7. Can you see light (blower removed) from the air side of the furnace?
8. How does the crack compare to the burner openings and the flue opening on each end of the heat exchanger?

 Read more that I have written on this subject at the freegas page
 and the question page .
To find out more about fan and blower motors click here
To find out what controls the fan in the heat mode or proper settings
To get parts look to Grainger or Johnstone.
To solve a problem use the link to the form below.
 Scott Meenen