Safe-exhaust water heaters are gas- or oil-fired units that are power vented. These systems completely isolate the combustion gases being exhausted from indoors. As a result, combustion gases cannot spill into the home. The safest systems also include a dedicated source of outside air for combustion. In contrast, less efficient gravity-exhaust natural gas- propane- or oil-fired water heaters 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.