From 1997 to 1999 Lennox had a pulse furnace testing and replacement option on all furnaces from the 14 up to the 21. These are a very well built unit, but with one problem. The pulse vibration causes breaks in the joints of the heat exchanger. These leaks have the potential to cause carbon monoxide leaks into the living space of the homes. These units need to be pressure tested every four years and most of them are due this year. The heat exchangers have a life-time warranty Lennox does not cover the cost of changing one out.