I have a goodman gmp-075 gas furnace. I had a home inspection done by some jerk! He noticed a crack in the sheet metal on the front of the exchanger. It was under warranty so I had a service guy come replace it. Seem the exchanger was fine but noticed my limit switch was shutting the furnace down, but coming right back on. He said my unit was over sized. I was a union fitter for 13 years. After 6 hrs of him in the basement and calling Goodman they decided that because the unit was over sized it wasn't cooling fast enough because not enough branches, and heat not escaping fast enough. MY QUESTION is... If he puts an instant read thermomerter in the exchange (which is where my limit switch is) , and it never get hotter than 140 in the exchanger....something is still not right! I took his explanation of not enough air being remove because of an over sized unit, but it still isn't right. Nothing is blocked, new limit switched was replaced, every thing is wired correctly, exchanger internal temp is still 20 degrees below the limit switch.

I may have not made myself clear about the limit switch. I had my heat exchanger replaced. The furnace keeps shutting down because of the limit switch. The tech is saying it has probably always been doing this, but I didn't notice, because it comes right back on. The fan continues to run, so if I am not watching it, I don't realize it is happening. He replaced the limit switch, but it is still doing it. He says it is because my unit is over sized. I find this hard to believe, because the temp (when using an instant read thermometer) is 140 in the exchange. 20 degrees below the 160 it is supposed to shut down at. Something still does not sound right. New limit switch was replaced, all wiring is correct, and air flow id good.