The next step, at least for me. Is to check to see if the furnace is really
over heating. I've seen a bad high limit switch. In that case, the furance
kept shutting down, but the furnace and the ducts never really did get hot.
So, I'd want someone (maybe the tech, maybe yourself) to see if the duct
gets very hot, where the duct comes out of the furnace. If dirty evaporator
is the problem, then the top of the furnace will likely be too hot to touch.
Most of the high limit switches I've worked on, run about 180 to 200 when
they trip off. And that's too hot to put your hand on.
Please go to the furnace and (very carefully) try to touch the grey metal on
top of the furnace, where the heat goes to the house. Is it burny hot, luke
warm, or room temp? That will tell us a lot. Also feel the top inch or so of
the furnace, where it meets with the ducts. Is that burny hot, luke warm, or
Christopher A. Young
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