Abstract: An improved double walled prefabricated fireplace apparatus insertable into a pre-existing masonry fireplace in a building for heating a room. The apparatus is provided with an air manifold within the apparatus adjacent the combustion chamber and having opposite mutually spaced walls through one of which an inlet for room air is provided and through the other of which an inlet for fresh outside air is provided. The manifold is further provided with an outlet for enabling air to flow to the air heating chamber between the double walls. A manually adjustable air flow control means is provided slidable within the manifold between two positions, one blocking the flow of air through the room air inlet and leaving open the fresh air inlet, and the other blocking the flow of air through the fresh air inlet and leaving open the room air inlet.