A low cost prefabricated fireplace is provided with an open-ended fireplace box which requires no outer sheet metal shroud or housing as employed in the prior art sheet metal fireplaces. The novel fireplace box has at least five walls which consist substantially of insulating reinforced ceramic fiber (RCF) material and a binder. A modular burner system is removably mounted on the bottom wall of the fireplace box and a floor panel is mounted on the modular burner system forming an air chamber below the floor panel in the fireplace box. A decorative surround trim is attached to the open end of the fireplace box which serves to support fixed or operable glass doors. A log set which may include a log burner is mounted above the floor panel which supports a decorative log set. Apertures are made in the sides or top of the fireplace box to accommodate an air supply and/or an exhaust stack. The fireplace box serves as a base unit to which other fireplace components are attached, connected or supported so that the need for conventional sheet metal is substantially eliminated. The resulting fireplace can be completed in many different configurations using a common fireplace box which needs no additional surrounding insulation for installation in a room.