Direct-vent fireplaces burn natural gas or propane, not wood. But direct-vent models are different from conventional gas-fired fireplaces, which require a through-the-roof Class B flue to carry away hot combustion gases.
Direct-vent fireplaces burn efficiently, extracting most of their heat from combustion gases and sealing the combustion off from interior rooms.
A direct-vent model has a glass door that is securely sealed to prevent leaks of combustion products into the room. Air to feed the flame enters from outside, and the relatively cool combustion gases are exhausted through the wall-mounted vent (if you prefer, you can also vent them through a roof).
Most of these units have passageways that direct room air around the firebox/heat exchanger and then return it to the room. Some come with fans to boost this circulation; with others, fans are an option.
These fireplaces garner efficiency ratings that approach those of gas-fired furnaces—in the 78 percent range. (This means they convert 78 percent of the fuel’s potential heat to usable heat.)