Natural vent, or B-vent, gas fireplaces and stoves use the air that is already contained inside of a room for combustion. Direct vent systems use indoor air for combustion. They take air to burn from the inside of the home.
Gas vent fireplaces exhaust their fumes through the roof via a pipe. This pipe consists of an outer pipe that draws combustion air from the inside, and an inner pipe which exhausts fumes. You have the option of installing either vertical or horizontal vents for the system. Direct vent fireplaces are made out of metal, and do not need a foundation or chimney. They usually burn with natural gas or propane. They are a direct competitor with natural vent gas stoves and fireplaces.
Natural vent systems are less efficient then their direct-vent counterparts. Heat often escapes from natural vent system, and their design permits back-drafting. This means that the air inside the room may pull gases back into the house. Direct vent stoves have glass doors which prevent the leakage of fumes into the house. Some also come with fans to improve circulation. B-vent furnaces do not have this feature.
Ease of Installation
Direct vent fireplaces work well in new, well insulated houses, as well as houses without chimneys. This is also true for B-vent fireplaces.