Natural Draft (Class B) Venting
Natural draft hearth appliances take in combustion air from inside the home and vent products of combustion outside the home.
Direct-vent appliances draw combustion air from the outdoors and exhaust its combustion products to the outdoors, eliminating the need for a standard chimney system. A glass panel in direct-vent units is critical to keeping the combustion system sealed from the home, maintaining high efficiency and indoor air quality. Direct-vent piping can go out the wall directly in back of the unit or terminate vertically above the roof. Various models differ. Check with your retailer.
Currently, the majority of gas fireplace sales are direct vent units.
Unvented or Vent-Free
Unvented and vent-free refers to the kind of hearth appliance that draws its combustion air from inside the home and is designed to burn so efficiently that it eliminates the need for venting.