Furnaces Not Flawed To the Editor: I read with great interest the article in the August issue by Gary Nelson on the flaws in superinsulated construction. In particular I was shocked to read of the eight condensing furnaces with corroded heat exchangers. As a Lennox dealer I have been installing The Lennox Pulse combustion furnace since 1982. Since then we have installed over 150 of them with not one heat exchanger failure. In fact, Lennox has extended the warranty from 20 years to a limited lifetime warranty on the heat exchanger. Based on our experience the furnaces that failed in that study must have been installed improperly or were simply inferior products to begin with. Ray Bennett Frontier Fuel Savers East Whately, Mass. Of the