I can not emphasize this enough …

Small heat exchanger cracks DO NOT cause carbon monoxide
to be picked by the indoor air stream.

This applies to residential gas furnaces with duct work connected and an air-conditioning coil on top. 

It does NOT apply to furnaces with open discharges (blowing wide open) with no duct work.  It does NOT apply to duct furnaces, or horizontal furnaces with no air-conditioning coil on the discharge.  In these cases there is a small chance that some undefined amount of carbon monoxide may pass from the heat exchanger to the indoor air stream. 


Example of Heat Exchanger Failure (5:24)

Note that this is a Janitrol furnace with NO air conditioning coil on the discharge, yet the blower pushes air INTO the heat exchanger with enough force to overcome the power of  the draft inducer fan.  This is how the flames roll out the front of the heat exchanger.  It is also why “roll out” safety switches are installed by the burner openings.

See “GrayFurnaceMan‘s” Youtube Channel for more instructive videos. 

This is what happens every time the blower comes on. (The main blower pushes air INTO the holes in the heat exchanger.) There is NO way for air from inside the heat exchanger to push itself OUT to the indoor air stream. The mechanic explains that the draft inducer pulls air through the furnace, and does tell the viewer that the roll out switches help protect the furnace.

Yet, hundreds of other web sites that tell you that a heat exchanger crack causes carbon monoxide because the burned gasses from inside the heat exchanger can move to the indoor air stream? It’s not true, and has not been true for the last 25 or 30 years, when duct work and air-conditioning coils started being mounted on the furnace discharge.

Example of Contradictions with the Evidence in Plain Sight

This video clip clearly shows holes in the bottom of a tubular heat exchanger. This is a “draft induced” burner, which means there is a small fan that SUCKS air through the heat exchanger and PUSHES it up the flue. The inducer fan is sized to pull the amount of air required by the burners at full-fire. No matter how big, or how small the hole or crack would be in the heat exchanger tubes, the draft inducer will always insure that the burned gasses travel through the heat exchanger tubes and up the flue.

The front of the furnace shows NO visible signs of heat damage, melted wiring wiring and there were no lockouts because of the rollout switches.  The technician hints that he could tell something was not right, but that the furnace continued to operate.  In fact he points out the locations of the rollout sensors (heat sensors) and notes that they are not in front of the burners that were not firing correctly.

Even with the draft inducer on the burner, the technician tells the viewer that it is very important to have the heat exchanger thoroughly checked.  He mentions that the holes in this heat exchanger didn’t happen all at once.  It probably took years for them to get this large.  Yet, he says this is a big problem and “this could kill the whole family”.

Think about that … the holes in the heat exchanger tooks years to form, yet they pose such a dangerous threat that they “could kill the whole family”.  That sounds like a contradiction in observations.  On the one hand, the holes take years to form, on the other hand they are an immediate threat.  The holes will affect operation and safety, yet they are so difficult to identify and diagnose that the technician had to pull the furnace to inspect the heat exchanger.

Holes like the ones on this video always cause burner rollout when the the furnace is fired and the indoor blower comes on.  When you watch the furnace run through a cycle, it is fairly easy to see the burner flames change shape and start rolling out the front. 


 Upflow Furnace with Atmospheric Burner and BAD Heat Exchanger

This is a good example of what a “flame disturbancelooks like in an older furnace. Note that the “test” for the heat exchanger is whether the blower causes the burner flames to waiver or blow around. Again, this means that the blower is pushing air INTO the heat exchanger. Since there is never any pressure created inside the heat exchanger, there’s nothing to force burned gasses into the blower’s indoor air stream. So the argument that a cracked heat exchanger causes carbon monoxide to enter the indoor air stream is false. If the burners ever created carbon monoxide, it would travel up the flue with the rest of the burned gasses.


Problems with Partial Blockage of Secondary Heat Exchanger on a 25 Year Old High Efficiency Furnace 

Here’s a good illustration of why older high efficiency furnaces should be replaced.  The secondary heat exchangers eventually will become blocked with residue from the products of combustion.  Although it is possible to clean many of these style furnaces, some of them are constructed with welded or inaccessible secondary heat exchangers.  By the time you pay to have the furnace cleaned and re-assembled, you might as well purchase a new high-efficiency furnace.

The residue build up is due to deposits from the condensing flue gasses and took 25 years to accumulate.  Overall, that’s a pretty good track record, even though one section of the heat exchanger probably rusted shut.