Gas boilers burn natural gas to heat water and then distribute heat to the home via radiators, a duct system, or radiant floor heating loops. High efficiency gas boilers 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 boilers optimize efficiency with a condensing process that extracts heat from exhaust gases with a secondary heat exchanger. High-efficiency gas boilers meet ENERGY STAR requirements for efficiency. Ultra-efficient gas boilers meet or exceed the criteria for ENERGY STAR’s “Most Efficient.”