Abstract: Outside combustion air is directed through a conduit into a special manifold positioned on the hearth of an existing fireplace. The manifold extends laterally across the fireplace opening and is positioned to block passage of inside room air through plural air supply slots in a door framework which lead to the firebox of the fireplace. In one form, a channel member secures the upper portion of the manifold to the door framework. In another form, plural mounting brackets attach the manifold to the framework such that the weight of the manifold causes a seal carried by the manifold to bear against the framework and substantially seal the space between the manifold and framework.