Just read your message. Your temperature rise is 135-75= 60 degrees F. Be careful drilling holes for the thermometer, try not to hit the airconditioner coil. You measured the temperature above the A/C coil and you got a reading of 135F.
Perhaps there is lint build-up underneath the A/C
coil. The High Limit Control is located near the tailpipe right after the combustion chamber. This
tailpipe glows orange from combustion.
If the high limit reaches 200F it should automatically reset. If you set the high limit to
220F the furnace will work? Is that what your saying? If thats the case, then that's what I would do.
Again I would like to stress the idea of getting ahold of a Lennox Dealer and checking this furnace out for heat exchanger failure if you have not already done so. Why put more money in repairs in this furnace if potential of heat exchanger leakage. Go to www.davelennox .com and enter your zip code.
If furnace checks out okay I would then replace High Limit if required. Probalby around $60 for the control. If furnace failed the pressure test for the heat exchanger, Lennox well replace for free the furnace (less Labor, taxes, installtion material required). The new model would be G51MP100Q3 it doesn't have "pulse combustion" and a whole lot quieter and another 92% efficient.
I've installed Lennox Pulse furnaces when they first came out back in 1982. They had trouble with the heat exchangers failing. This is why I'm saying to having it checked. If money is a problem you can get financing through Lennox. Perhaps phone a few dealers and get the best price. In all sincereness hope my ramblings help.
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