Installing vent-free gas logs into a factory built fireplace (prefab) is a real controversy right now. The fact is that prefabs were meant for use with the damper open and the installation of vent-free logs implies using the prefab with the damper closed. When the damper is closed and the vent-free logs are being operated, there is a chance that the temperature in the flue system or at the top of the chimney can reach an unsafe temperature.
The fact of the matter is that there is not sufficient data at this point in time to say outright that vent-free gas logs in a prefab unit is unsafe, but some studies have shown that the temperature at the top of the chimney in a prefabricated unit does reach an unsafe temperature when vent-free gas logs are operated.
Vent-free gas logs come with other issues as well. Air quality and vent-free gas logs are a major issue. Professionals state that vent-free gas logs should not be operated when there are elderly or infants in the home because of the risk of decreasing air quality in the home. Although most vent-free gas logs come with an oxygen depletion sensor (ODS) some do not and there is always the risk that the ODS will not detect an unsafe drop in oxygen in the home. Odors are another issue with vent-free gas logs.
Generally vent-free gas log odor is linked to the presence of chemicals in the home such as house hold cleaners which get on the logs and produce the odor, but sometimes the vent-free system can just smell. Vent-free log sets usually have a distinct and unattractive odor for the first five hours of operation or so. Usually this odor goes away.
Vent-free gas logs are extremely controversial no matter where they are installed. I do not recommend the use of vent-free gas logs, although some folks are all for them. If you have vent-free gas logs or are looking to purchase a set, they should be installed in an existing, operable masonry chimney.