Vent free technology is the most efficient of the three options one can use in an existing masonry or pre-fabricated wood burning fireplace. It also gives one the total control over the damper position. Despite the common misconception, vent free log sets can be used as purely vent free (without a damper or with a damper closed), partially vented or fully vented. That is the opposite of what the vented log set technology has to offer.
When operated in vent free fashion, vent free technology is 100% efficient, making the money spend on gas/propane well worth the output in heat. Since any fire needs oxygen to burn and produces CO, CO2, water vapor and smell, vent free log sets will be using the room air for combustion and will emit the above mentioned by-products back into the room. This scares some people, but in reality using this technology is no different than cooking on a cook top stove.
When compared to a vented gas log set, vent free gas log sets are nowhere near as pretty or realistic, but they can be used as a secondary furnace if desired.
All vent free products have oxygen depletion sensors (ODS) built in them, which is a nice piece of safety equipment for the peace of mind. Should the ODS sense that the oxygen level in the room is approaching an unsafe level; the unit will turn itself off. That way there is no more heat, no more flames, no more exhaust, and, ultimately, no more oxygen being depleted from the room by the fire.
Due to their nature, vent free products cannot be installed in bedrooms, must feature 40,000 BTU's or less and comply with specific clearance requirements designated by the manufacturer and/or fire code.
Log sets are not the only option, when it comes to having a vent free technology installed inside of an existing wood burning fireplace or a vent free firebox. For people who prefer a modern look to an imitation of a conventional wood burning fire there are options of contemporary burners with glass and river rocks along with simple "linear" burners. Regardless of their different look these burners function the same way as traditional log sets, hence all the above mentioned characteristics, requirements and restrictions apply to them as well.