Safe-exhaust boilers are gas-fired units that completely isolate the supply of combustion air and combustion gas exhaust from indoors. As a result, combustion gases cannot spill into the home. In contrast, less-efficient gravity-exhaust natural gas- or propane-fired boilers draw combustion air from the room in which they are located and rely on an open flue to expel combustion exhaust up and out of the home. However, bath fans, kitchen hoods, and clothes dryers can potentially overcome the upward flow of exhaust in these non-sealed systems and draw combustion gases into the home.