Good information and questions, I also wondered why the limit switch was not tripping sooner, as I looked at the furnace, the limit switch is located at the front of the furnace and as the fan blows the air past the heat exchanger itís almost a direct passage through the heat exchanger through that center duct and past the limit switch, that center duct does not get nearly as hot as the one above or below it.
I changed five of the ducts in order to eliminate sharp turns, my thinking on the return air was to keep the basement door open to make sure the furnace was getting enough return air, the duct temp measured with a digital probe with the door open or closed is the same. Changing the ducts to allow better airflow did make a difference. It used to cycle on the high limit switch after about 3 to 4 minutes of operation and then after three times it would go into lock out so Iím making improvements but there is still something thatís not right, i have checked voltage, but will check again, do you think 6.5 amps indicates a problem? Donít have a way to measure cfm, the bathroom register that gets the hottest (180) and the shortest duct (18Ē) seems to have great air flow. 🙂
RG 2012 Excel 31 RSE Winslow