Direct Vent Gas Fireplaces are usually UL Approved room furnaces.  The gas valves tend to work with the same principle as a home furnace. You can hook up a room thermostat to them, but they do not need electricity due to the millivolt system. Blower motors can be powered by 110 volts, if needed. Direct Vent furnaces and fireplaces are designed to use only outside air. Typically you'll be in the 85% range on gas efficiency, which will calculate to approximately 12 cents per hour in natural gas.