B-vent appliances use a single exhaust pipe and draw combustion air from the room into the combustion chamber and up through the chimney. These devices are sometimes referred to as “natural-vent”. Direct vent appliances are sealed to the room. They draw their combustion air from outside and exhaust through a collinear or coaxial pipe. In either case, efficiencies on new appliances don’t vary much between the two configurations. Most new gas stoves and fireplaces use very small 3″ or 4″ diameter exhaust systems.