Question: How do I determine whether to use B-vent or Direct-vent for LP gas fireplace inserts? Answer: Depends on the exact model of insert. If your insert has one pipe coming off it, it's a "b-vent" and needs one liner. If it has two pipes, then it's a direct vent model. B-vents use room air for combustion and put the exhaust up the chimney. Direct Vents draw air down one pipe and put exhaust up the other. Because of the two-pipe system, Direct Vents are probably less likely to experience draft problems related to air movement in the home. They are also more effficient since they do not use air from the living space to burn.