Vented gas log set is the first technology that comes into one’s mind when converting an existing masonry or pre-fabricated wood burning fireplace to gas as it is the most realistic looking option of the three and the one that offers the most options and choices of style and size.
This technology is the least efficient of all the three technologies for existing wood burning fireplaces; it is only 20-30% efficient and requires that it is used with the damper wide open. Vented gas log sets are still two to three times more efficient than a straight wood burning fire since wood burning fires are zero percent efficient under the best circumstances.
Vented gas log sets require a fully open damper when being operated as they produce soot (not as much as wood burning fire would). In addition to that, the fire code requires that damper has to stay open (not necessarily fully) at all times, even when the logs are off. In more cases than not, this will equate in cold air coming down the flue and warm air escaping up the chimney during the colder months and vice versa during the summer. That’s why a lot of times glass doors are very nice* to have in conjunction with a vented log set.
There are more efficient vented gas log sets (up to 50%) known as radiant heat log sets that boast 30,000-40,000 BTUs of heat output. While being higher in efficiency, they still belong to vented log set technology and all the code requirements associated with it.
Log sets are not the only option, when it comes to having a vented technology installed inside of an existing wood burning fireplace. For people who prefer a modern look to an imitation of a conventional wood burning fire there are options of contemporary burners with glass, river rocks and even an assortment of geometric shapes. Regardless of their different look these burners function the same way as traditional log sets, hence all the above mentioned characteristics and requirements apply to them as well.
*NB: Nice vs. need! Glass doors are not necessary, but they will certainly help with drafty chimneys.