Abstract: An apparatus for improving the thermal efficiency of an existing fireplace is disclosed. The apparatus generally comprises a convection chamber having an open front and top, a bottom, a rear wall, and a plurality of side walls. The convection chamber is inserted into the existing fireplace and is spaced apart from the fireplace such that an air passage is defined under, behind, and along the sides of the convection chamber and the fireplace bottom, rear wall, and side walls respectively. The air passage draws room air to be heated under and behind the convection chamber, and discharges heated air into a room to be heated along the sides of the convection chamber in the air passage between the fireplace side walls and the convection chamber side walls.