Condensation produced by midefficiency gas furnaces in metal and masonry flue liners has brought about testing and debate in the masonry and gas heating industries. Under the National Appliance Energy Conservation Act of 1987 (NAECA), all residential, gas-fired, warm-air furnaces sold after January 1, 1992, must demonstrate an Annual Fuel Utilization Efficiency (AFUE) of at least 78% (midefficiency). Most furnaces able to meet this requirement have a fan-assisted design. Fan-assisted furnaces don't have a draft hood. Instead, they use a small fan at the flue outlet to pull combustion gases through the heat exchanger and deliver them at neutral pressure to the venting system. The fan-assisted furnace's elimination of dilution air (a low-humidity mixture of room air and flue gases pulled in through a draft-hood furnace) tends to increase condensation in the venting system. As a result, fan-assisted furnaces are less forgiving of variations in venting system design and may require changes in traditional materials and techniques.