Gas Fireplace Venting Options
Vented, or natural vent, uses a chimney for venting; it utilizes room air for combustion for burning and vents out above the roofline.
Direct vent is vented out of the top or the back of the unit and can be vented horizontally or vertically. A two-layer pipe draws air in from outside to create combustion and the other pipe removes wastes to the outside. It doesn't require a chimney.
Non-vented fireplaces, also called vent free or unvented, do not require a chimney, flue or vent. Consequently, they can be installed almost anywhere. Vent-free models have an oxygen detection safety-pilot, or ODS, which automatically shuts off the gas supply if the oxygen level in the room drops below 18 percent.