There are three classes of venting a) direct vent b) B-vent c) no vent
In direct-vented products we use either co linear or coaxial venting. In either case one part of the system is the exhaust and the other is the intake. The unit itself is a sealed combustion system and no air from the house is involved. The vents can go either through a sidewall or through the roof. (see photo below)
In B-vent (natural vent) the units have a draft hood which pulls air from the room to help dilute, thereby slowing, the draft. They also use room air for combustion. They are atmospherically vented with double-wall metal B-vent pipe. B-vent is only vented vertically to the outside, but it does not draw outside air for combustion. (See photo at right).
No vent units (see photo bottom right) reflects the ultimate in innovation and efficiency. Ideal for contemporary living spaces, high rises, lofts or any space where venting cannot be installed, the slim and modern design makes installation as easy as hanging a picture on a wall. They're less than eight inches deep! And because this fireplace approaches a 100% fuel efficiency rate, nearly all the heat produced is distributed directly into the living space.