What makes the difference between a vented and vent free set? Does one require a special fireplace? What can I do with one that I can't with the other? Vented and Vent free sets have their differences but first, let's talk about what is similar about them. Both Vented and Vent-Free sets require a fireplace that is suitable for wood burning. If it meets the requirements to burn wood, it can house a fireplace set of either type. Also, both provide similar levels of heat and ambiance. Aside from that, these types of sets differ in a few, but important ways.
1) Vent Free sets can be used with the flue closed
Vent free sets do not require as much air circulation as a vented set, which requires the products of combustion to be vented up a chimney. Vent free sets do not put out as much negative products of combustion as a vented set and thus do not require venting. As a result, more heat can be kept inside instead of escaping up the chimney. Unfortunately, this does not come without its downside.
2) Vent Free sets cannot be rearranged, they have to be assembled as instructed
By this I mean that the logs, stones, shapes, balls, or whatever vent free objects are a part of the set have to be arranged as specified by the manufacturer. This is because vent free sets have been tested only for specific arrangements that minimized negative products of combustion. If rearranged, then there's a risk that gas will move differently over the objects, not burn as it should, and create potentially harmful gasses such as carbon monoxide, but do not worry: the ODS pilot that is a part of these sets will shut the unit down if it does not detect oxygen, thus making it into a shutdown issue instead of a safety one. If sets are rearranged, then you risk your set shutting down mid operation and not working properly with vent free sets. Of course this disadvantage implies the advantage of Vented sets.
3) Vented sets can be arranged and customized
Vented sets do not have the strict arrangements as vent free sets. Logs and other objects can be shifted around to the owners content within the limits of the fireplace and the burner. The way gas moves and burns around the objects doesn't matter as much because the flue will be open and air will be drawn into the fireplace and pulled up the chimney, so the majority of negative products of combustion, if not all of it, will be removed from the house. Because of this, vented sets allow for greater customization options including larger set sizes, where vent free sets will not be larger than 30 inches.
4) Vented sets have a larger yellow flame
Vented sets can have higher BTU's and larger flames. The flame from a vented set will be more luminous and bold. Unfortunately it will produce more soot and dirty the logs and the surrounding fireplace faster than vent free sets.