My furnace guy said the heat exchanger of a unit has cracks. I also saw it; but it looked like a crack on the a little bit rusted surface, but i did not see a hole or anything like that. The guy put CO detector at the flue and the CO level shot up at 200 ppm and came back down at 25-30 ppm which I think is normal because that is what the flue is for. He said that is too high, and he says he sees usually 5ppm or low level. Potentionally, it can leak into the main airstream. But there is no CO in the air stream into our house. And then, he said I need to replace the whole unit or the heat exchanger. Is what he said reliable?
What the HVAC guy said was correct. The only way that you can inspect 100% of a heat exchanger would be to remove it from the furnace. This isn't practical, so what we do (I'm a home inspector) is shine a light up the exchanger and look at it with an inspection mirror, which is a 2x2 mirror on a telescoping antenna. Rust isn't something you should see on a heat exchanger, that's where it will be at its weakest point. In an inspection, you're probably not going to see more than about 10% of the exchanger. If the exchanger is cracked, it's very likely that this is indicative of the condition of the rest of it. So, if you saw some cracks, there are likely 9 more for every one you saw. Not only is this bad, but it's dangerous. You shouldn't see CO levels of 25-30 ppm in a home. That is an extremely high number. It should be less than 5 ppm at any given time. What he said about the CO leaking into your air is correct if there is sufficient negative pressure in your house. That can vary whenever a door or window is closed, when dampers are shut, etc. You don't know that there isn't CO in the air you're breathing. The CO detectors will only pick it up if there's more than 70 ppm for over 8 hours. At that point, you're already going to be showing symptoms of CO poisoning. The detectors in the store are set way too high. The ones that pros use will go off at 35 ppm or less.
If you don't believe him/her get a second opinion. When it comes to heat exchangers don't screw around, its not worth your or your family's lives. When the exchanger needs to be replaced if the furnace is older it'll be cheaper in the long run to invest in a new high efficiency unit. The sooner the better.
why not get a second opinion? ask friends, neighbors, and family for referrals for a furnace repair guy that they trust and use. have him check it out for you and verify whether or not it's shot. also you could check with the Better Business Bureau on your furnace guy. it doesn't make good business sense for him to lie to you about it though. but since most people don't have a clue about it, it's scary, so they do it. it is no different than a repair shop scaring a woman about a car repair that isn't needed. myself, i have a contract with an oil company because of the service techs. i might pay a few pennies more for oil, but the service guys are why i am with this company. when you think what a heat exchanger is and how it works, it's not something to take chances with. if it does fail on you, the results could be deadly for you and your family. one more thing, how old is your furnace? the new ones are more fuel efficient and a new one could save you more in the long run than repairing the old one. and just in case, if it's gas fired, have the gas company check it out. good luck, and don't wait on getting it checked out.