Abstract: A chimney, waste-heat, collector includes a plurality of elongated sections, coupled end-to-end and lowerable successively down a chimney. The collector comprises at least one vertical, up-flow, conduit and at least one down-flow conduit within the chimney forming a closed loop system with a pair of horizontal conduits extending out the ash pit opening to a heat storage and dissemination means. The couplings are threaded unions for ready assembly and the sections are preferably of right angular, or rectangular cross section and of thin wall stainless steel. A pump circulates liquid around the loop when the furnace is operated.