Converting from electric to gas does complicate things somewhat. Gas needs a vent to keep the exhaust fumes from building up around the appliance where they can create serious problems. Venting comes in two styles B or Natural Vent and Direct Vent. A B-vent or Natural vent system exhausts fumes that are created when combustion air is taken into the appliance from the room where it is located. A Direct-vent systems is a closed system. It delivers outside combustion air to the appliance and vents that air back to the outside the building. When comparing prices of appliances offering a choice between these two venting systems, expect the Direct-vent to cost more.