High efficiency gas furnaces use sealed combustion with two ducts that fully isolate the combustion process. This includes one duct that draws combustion air from outside the home and one duct that exhausts flue gases directly outside. Thus, sealed combustion eliminates the possibility of back-drafting harmful flue gases back into the home or consuming too much oxygen. These furnaces optimize efficiency with a condensing process that extracts heat from exhaust gases via a secondary heat exchanger. High-efficiency gas furnaces meet ENERGY STAR requirements. Ultra-efficient gas furnaces meet or exceed the criteria for ENERGY STAR’s “Most Efficient” designation.