I have a Trane xe 80 furnace that continues to overheat. When I came home the other day the house smelled like the smell from a portable heater you sometimes get when its first turned on. From then on it would not work properly. It ignites, burns, and blows normally for a minute or two and then shuts down. Then giving me an error code for the high limit switch. I have had the switch checked and it is still good. I have cleaned the flame sensor, the inducer fans, the burner tubes, replaced the filter, ran a leaf blower up the flue to make sure it was clear, cut power to reset the furnace and still no luck. I checked the temperature with a cooking thermometer at the base of the flue and it IS overheating. I checked the heat exchanger for any visible cracks, and although I wouldn't call it a thorough inspection, I did not find any visible cracks. I noticed the flames from the burner tubes seem to join together in between the main flames that shoot into the heat exchanger. Is that normal? The burner tubes seemed to be dirty, but in good shape when I cleaned them. At this point I am unsure what to do next. Any suggestions before I call in the service guy?

While you attempting your own fix I'd get a carbon monoxide detector going in this home. When you say overheat do you mean the house heats up too much or some limit switch kicks out

The limit switch opens due to the overheating. Its my understanding that the function of the high limit switch is to shut the system down when it exceeds a certain temp. That's the error code I get. However, its not the problem. I will take your advice on the carbon monoxide monitor. Good idea, even though the system is not functioning. Any other ideas as to what might be causing the overheating? My guesses at this point are a cracked heat exchanger or a problem with the blower fan being really dirty or obstructed somehow. Seems like a pain to access the blower fan so I haven't had the time to see what's going on inside there. I'm stumped.

Sometimes this can happen if the fresh air supply is blocked, do you know where your intake is located? Check it

Make sure the returns aren't blocked, most high temp issues are air related. If you do have a cracked exchanger, check your model for a recall. Do not operate with a cracked exchanger if that is the issue.

Good call. I checked the intake from below, but didn't see anything blocking it. I suspected there may be some sort of filter so I climbed up and looked closely. Sure enough there is a piece of mesh behind the vent and most all of the mesh is filled in with gunk. I don't have time to do it tonight, but I will tomorrow and let you know how it goes. I bet that is going to do the trick if I haven't burned out the blower motor too much. Good lesson. Thanks for the good advice. Keep you posted.

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Originally Posted by ma0641

Make sure the returns aren't blocked, most high temp issues are air related. If you do have a cracked exchanger, check your model for a recall. Do not operate with a cracked exchanger if that is the issue.

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Returns are clear. Agreed, won't run it with a cracked heat exchanger. Going to replace the dirty intake mesh filter and see what happens. Thank you.

[QUOTE=ballengerb1;3764039]Sometimes this can happen if the fresh air supply is blocked, do you know where your intake is located? Check it[/QUOTI
I cleaned the mesh filter behind the intake vent and it helped, it stayed running for a half an hour before overheating again. I will try to clean it better, but to replace it I would need to remove the siding around the vent which I would rather avoid if possible. I cleaned it with bleach and it worked pretty well.I fear the blower has gone bad, or at least is obstructed in some way. The blower comes on and runs, but the air coming from the returns is weaker than it used to be. The smell I had in the house when it started could have been the blower motor over-working itself due to the clogged intake. I guess there could be an obstruction somewhere between the blower and the returns, but that seems unlikely. Is it possible for the blower to still run but with that much less power? A new blower with the assembly is more than it costs for a new furnace less the installation. Any way to test the blower without removing it? Ideas welcome.

Try removing your furnace air filter just to see if things improve.

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Originally Posted by ballengerb1

Try removing your furnace air filter just to see if things improve.

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I removed the air filter and it ran a little longer - 45 minutes, with filter 30 minutes - and air flow seemed better. W/o the filter was about the normal air-flow it usually produces. When I put the filter back in, it seems to be just trickling out of the return. When looking at the blower assembly I noticed there is another filter that sits along side the assembly. It's a metal filter. I tried to remove it to check and clean it, but it seemed like it wouldn't come out unless the assembly was first removed. At this point, I think my next step is to pull the blower and assembly to check for any obstructions, clean the fan and see if its running properly. Not what I wanted to do, but I am out of other ideas. I read a post from someone with the same problem that said the material they used for a noise barrier around the fan got sucked into the squirrel cage causing it. If I am overlooking something simple, I'd be happy to hear from you.