Abstract: A gas operated fireplace assembly is comprised of a plurality of modular components, including a casing, firebox, burner module and combustion air-intake module. The casing has an open front and is positionable in an enclosure adjacent a room with the open front facing the room. The firebox is insertable into the casing through the open front thereof and is removably mounted within the casing such that the firebox is removable therefrom without having to remove the casing from the enclosure. The casing can be installed separate from the other modules (e.g., during the "rough-in" phase of construction) and the other modules installed at a later time. Both the casing and firebox have respective top panels which are sloped at approximately 25.degree. to accommodate both a vertical and a horizontal flue configuration while maintaining the front-to-back depth of the fireplace assembly within acceptable limits.