Newer, more efficient gas furnaces extract more heat from the flue gases before they are vented into the chimney. Although these furnaces conserve fuel, they also cause corrosive condensation problems. When flue gases enter a chimney at just over the dew point temperature, they quickly cool to below the dew point, then condense into water and corrosive acids. The acidic condensate eats through stainless steel, aluminum, and galvanized flue liners and leaks through the butt joints of clay flue tiles. It's not a little problem. The problem is especially acute in masonry chimneys with flues too big for the efficient new gas furnaces. SEEKING SOLUTIONS A good solution seems simple enough. If these new furnaces are vented into smaller, 4- or 6-inch diameter tile flue liners, most of the effluent will stay above the dew point and exit the chimney in vapor form. Shiplapped or belled joints in these smaller flues will keep any condensed effluent inside the liner. The gas utilities and gas furnace manufacturers seem blind to the obvious advantages of clay tile liners in masonry chimneys. The National Fuel Gas Code (NFPA 54 and ANSI Z223.1) currently is being revised so as to deemphasize, if not prohibit, masonry flues in favor of "listed" metal and plastic liners.