A condensing gas furnace recovers energy normally lost by conventional furnaces. In the standard forced-air gas furnace, some of the heat energy produced by burning natural gas goes up the exhaust vent with hot combustion gases. That’s the reason standard gas furnaces have always been limited to an AFUE (annual fuel utilization efficiency) rating below 90 percent. A condensing gas furnace incorporates an extra heat exchanger to extract heat released when combustion gases condense. Condensing models break through that efficiency ceiling and offer AFUE ratings of 90-98 percent. However, efficiency ratings, household comfort and even safety assume an installation performed according to industry standards. Here’s what to watch for in the installation of a condensing furnace: