Vent-Free. These units have no flue, so they can be installed anywhere—even hung on a wall like a flat-screen TV. They're up to 99 percent efficient because all their heat stays in the room—as do all the pollutants. Because of concerns about air quality, indoor units are fine-tuned to burn extra clean, and their heat output is limited to 40,000 Btus. They have an oxygen-depletion sensor that automatically shuts them off if a room's oxygen level dips below 18 percent (20.9 percent is normal). Even so, vent-free units are banned in California and some U.S. cities.