Natural draft furnace from 1987 Pilot would not light Removed pilot burner, disassembled, and cleaned Reassembled Furnace now operates but cycles on the high limit Increased the blower speed, still cycles on high limit Blower is very dirty. Fully expect air conditioner evaporator coil on top of the furnace to be plugged with dirt, as well. These are the biggest causes of furnaces to overheat and trip the high limit Performed a combustion analysis on the furnace Readings are way beyond acceptable for a healthy heat exchanger There are large cracks around the burners on the face of the heat exchanger Currently there is no carbon monoxide leaking into the house, but the risks are extreme in this case due to the poor combustion and the large cracks. Furnace needs to be replaced.
