Vent free appliances are manufactured so that they do not release soot or CO when burning. The burner is designed to facilitate and optimal fuel ratio, with no obstruction to the flame and the release of only carbon dioxide and water vapor. It is important to note that they do use oxygen from within the room for combustion and as such, they do not work well in smaller spaces and high elevations, due to the lack of available oxygen.
Collin C. - NFI Master Hearth Professional
on October 25, 2013