It's an industry standard: if a furnace has a cracked heat exchanger, it gets replaced. The American Gas Association has even put this in writing - they say "Any visible crack or hole is reason for requiring replacement of the heat exchanger or furnace." When I inspect a furnace and I find a cracked heat exchanger (and I find a lot of them), I always say to replace it.
So what's the big deal with cracks or holes? A cracked heat exchanger could allow exhaust gas from the furnace to contaminate the household air with carbon monoxide. In order for this to happen, the furnace must be producing high levels of carbon monoxide AND the exhaust gas must be mixing with the household air. For a good example of a hazardous heat exchanger, check out the photo below showing a large rust hole in the heat exchanger of this high-efficiency furnace that was only ten years old.
Cracks, on the other hand, I'm not so sure about. With the majority of the cracked heat exchangers that I've seen, I've always been curious how the exhaust gas from the furnace could possibly leak out of those tiny cracks enough to contaminate the househouse air. Of course, what I'm curious about doesn't matter... but sometimes my curiosity gets the best of me, and I have to find out for myself.
So I did. I took home a furnace that had a cracked heat exchanger, tore it apart, and removed the heat exchanger cell that had the largest cracks. You can see the cracks for yourself below - click on any of the photos for a larger version. This first photo shows the cracks as seen from inside the heat exchanger cell - this is what we saw during our inspection.
These next two photos show the cracks from the exterior, or blower side of the heat exchanger cell. This part of the heat exchanger is usually not visible during the course of a home inspection.
I wanted to see if water would leak through these cracks, so I doused the outside of the heat exchanger and looked inside for any signs of leakage. Nothing. I've heard that penetrating oil, such as WD40, will get through the cracks, so I tried that next. Nothing. Feeling pretty disappointed at this point and determined to get some results, I filled the heat exchanger with water. I laughed like a mad scientist at what happened next.
Water began to leak out of the factory seam in about ten different places, but the crack never leaked.
Why does this tiny crack mean the furnace should be replaced? Because the American Gas Association says so. Don't argue.
Apparently, when heat exchangers get very hot the metal expands and the cracks open up, allowing air to leak in to or out of the heat exchanger. Just because my test didn't allow any water to leak doesn't mean that this furnace was safe, and it doesn't mean that a different furnace will behave the same way... but I sure found it amusing.
Maybe I need to get out more <glaven>