i recently had my n.gas furnance gas line caped by the local gas co. i was served an paper that basicaly said that i was not to opperate the appliance untill the problem was corrected. the problem it seems is that there was soot collecting right under the heat exchanger.i took a close look at the inside of the heat exchanger... and it seemed to have quite a lot of soot clinging to the burner side of this element.is this a sign of a broken heat exchanger? i had a service company out to look at the appliance and they told me that it was and that a new furnance was in my future. i just want to now if i'm being taken advantage of or this is reasonable .
Do you own a shop vac. Turn off the Furnace and Vaccume out the soot, use a flashlight to make sure the bottom of the burner hasn't rusted thru. You may want to check the flow off the injectors(this should be done by a pro) If a lot of soot is building up there may be a problem with the exhaust being blocked with soot also.
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Even a cracked heat exchanger can be replaced,we do it all the time,but the cost might exceed new furnace price.But you cant condemn a heat exchanger simply because of soot because there are too many other causes of incomplete combustion. There are jakeleg service techs out there who wont even take the cabinet off the furnace then tell you you need a new one.The only real way to condemn it is to find the crack if its there.And sometimes it can be difficult and cost a lot of labor hours. If your technician showed you the crack,then you can assume it needs a new heat exchanger (or new furnace).If not,well maybe you should buy a second opinion.There's some serious $$$ at stake.
An easy quick test is to use a long fireplace match, lite it, and then stick in the middle of each chamber cell while the blower is running. If there are any pin holes or cracks in the chambers, then the flame will flicker and move around due to the air from the blower entering the holes that may have developed. This is also a good time to consider the credit back offered for having a new efficient furnace installed, plus the amount of money you will save on your gas bills if you were to purchase one of the better ones like a 90+ unit.
Soot in the combustion chamber can be caused by misaligned burners, too, where the flame is touching the chamber side and cooling the flame. The poster is right about the difficulty of finding a crack or hole in the H.E. and there are instruments that can check for efficiency, flue gas temp, stack temp, CO test. I think the only sure way it to take it out. There is one sign of a crack and that is if the flames blow around when the blower fan is on. The burner alignment would have to be corrected first, then you could see if the flames move. That's would I would try to do is have the soot cleaned and align the burners.
