Gas fireplaces do not require a chimney, but most options require venting using a pipe or flue. Vented gas options include B-vent and direct vent, though you can also find ventless options. They offer the following differences: B-vent, sometimes called natural venting, make use of the air inside the room for combustion. They use a single pipe to vent the fumes from combustion outside. These options need to vent vertically, meaning they cannot vent through the wall. They must vent through the roof. Direct-vent gas fireplaces use outdoor air for combustion. They use a double vent pipe whose inner chamber exhausts the fumes from combustion while the outer chamber draws in fresh air. They can vent vertically or horizontally. Vent-free, or ventless, gas fireplace options do not require venting. Instead, they cycle the products of combustion back into the room in an amount deemed within safe parameters. However, they still emit fumes and can deplete the oxygen in the room. Consider investing in oxygen-depletion sensors to sense when oxygen levels drop too low and crack open a window to replace the oxygen lost. Ventless options may not work in all regions. Check local codes and laws before purchasing.