Abstract: A heat exchanger for optimizing the room heating efficiency of a typical fireplace which comprises a plenum including a radially extending housing having sloped top and bottom walls connected at a closed apex. The housing includes respective air inlet and outlet passages and means for removably mounting the plenum a predetermined distance above a fireplace hearth. The means locates the rearward end of the air inlet passage provided in the plenum angularly above the hearth and the apex of the plenum housing defines a flue passage with the rearward wall of the fireplace for expulsion of effluent gases. The plenum and means are discrete and separable from the fireplace.