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I had someone come in to inspect my furnace and was told my heat exchanger has a crack and the whole system should be replaced ASAP due to carbon monoxide risks. It's 16 years old but I haven't had any issues with it. So I'm wondering if it's a scare tactic. I already have CO alarms around the house.

In your opinion, does the first picture below (left side of exchanger) look like a crack? The second picture is the right side, which still looks really good.

left side of heat exchanger, showing the "crack"

right side of heat exchanger

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    A crack there shouldn't make a difference, otherwise those two giant holes on each side of it would be a problem too. That's not to say that there's not other cracks, just that the one in the photo shouldn't be a problem. – Tester101 Nov 12 '16 at 16:43
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I had a similar experience. I did not trust the tech's report and felt like I was being setup for a replacement instead of a repair.

The tech's report mentioned a smoke test. More research online revealed a match test. My furnace has flame injectors. Remove cover to injectors. Turn on heat. Observe flame. Any orange? Even intermittent? Possible crack. Turn off heat. Turn on fan (only). Turn off gas at valve by furnace. Ventilate to eliminate gas traces. Light a match and hold it up to each heat exchanger opening. Does the flame move as though air is passing over it? If so, exchanger is cracked and should be replaced.

I found a new exchanger for my 19 yo Goodman 92% furnace online for $100 + $40 shipping. Used Permatex Red to replace gasket to secondary heat exchanger. It was quite an effort.

(Side note: had to replace control board before I could perform above test.)

All summed up: $80 tech visit, $75 board, $140 heat exchanger, $20 supplies, gas sniffer (forgot how much - worth peace of mind - good tool for any DIY'r) Experience: priceless; when I finally replace this furnace I will do it myself Alternative: $800+ new furnace (DIY) or $$$$$$$ by HVAC Inc.

  • Great story. I caution anyone trying this themselves to (a) be VERY aware of the carbon monoxide risk (I had a repair recently and my HVAC guy said he can keep things going for years with occasional repairs, as long as there is no CO problem and told me to get more detectors, which I did) , (b) working with gas (e.g., any pipe repairs or furnace replacement) is potentially FAR MORE DANGEROUS than water/sewer plumbing. While any handyman or DIY can, if they know what they are doing, do a passable job on water/sewer repairs with little real risk, I would only get a licensed professional for gas. – manassehkatz Jun 20 '18 at 15:45
  • I had a furnace that was fine, until it got good and hot, then it started pouring out CO. A gas company rep verified this with a handheld CO meter, and the landlord swapped it out pronto. Presumably some crack got wider as it heated up. – Bryce Jun 20 '18 at 17:51
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    Word of caution on CO meters: the very sensitive ones have an exposure limit and shelf life. This means once the cartridge is unsealed the shelf life starts ticking - it's about 1 year. The exposure limit is harder to gauge - if it sniffs a high concentration of CO for a long time, then the cartridge expires faster then if used occasionally at low concentrations. The cartridges are expensive. I like the idea of the gas rep bringing his fresh sniffer out. I would gladly pay for that service. Also, the increase in CO at higher heat suggests cracks opening up through expansion. – Msquared Jun 20 '18 at 18:16

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