For masonry or factory built fireplaces you have two options:A sealed gas insert will slide into your existing firebox, providing supplemental heat and ambiance. These units require an existing chimney and include a glass front. The alternate option is a gas log set or glass burner. This type of unit creates more of an aesthetic presence than heat. These units do not come with a glass front, but doors are available if desired. The burner is assembled and connected to a gas line within the firebox.