Abstract: A closed-loop heat-transfer system includes in combination an exhaust chamber for positioning in series with a chimney flue of a heating system wherein the exhaust chamber has an exhaust inlet and an exhaust outlet opening adapted for connecting the chamber to the chimney flue. The exhaust chamber includes a first heat exchanger having a fluid inlet opening and a fluid outlet opening and a coiled conduit positioned interior to the chamber interconnecting the fluid inlet and outlet openings. The heat-transfer system also includes a second heat exchanger for positioning in a location remote to the chimney flue, and the second heat exchanger also has a fluid inlet opening and a fluid outlet opening. A conduit couples the fluid openings of the first heat exchanger to the fluid openings of the second heat exchanger to form a closed circulatory loop for a heat-transferring fluid.