A gas fireplace doesn't produce smoke or odors, but its flames do create pollutants, such as carbon monoxide and nitrogen and sulfur oxides. Vented fireplaces send these emissions outside, while vent-free ones direct them into your living quarters.
Vented. The safest, most efficient venting method is a two-pipe system known as direct vent. One pipe feeds fresh outside air for combustion to a sealed firebox and the other takes away the combustion fumes. The pipes can go up a flue or straight through an opening
in an exterior wall, as shown. These units are 70 to 85 percent efficient. A less common option, called B-vent, has a single pipe to carry away exhaust; the flames get combustion air from inside, making them about 50 percent efficient. B-vents should have a sensor to douse the fire if exhaust gases ever reverse course and come down the flue. Direct vents eliminate that risk.